Introduction to Laser Instruments and Applications

When the laser was invented in 1960, it was amazingly, a solution looking
for a problem. While the laser's weapons potential was clear, most of the
uses of lasers that have changed the World were not foreseen even by the
so-called experts of the time. In this chapter, we touch on perhaps one
tenth of one percent of those where lasers are now indispensable, or at
least have the potential to be in the future.

But lasers are not the solution to every problem. There are
applications where lasers are not useful and probably never will be.
Among the short list of idiotic proposals for lasers are (in no particular
order): grass and tree trimming, insect extermination, and advertising
on the moon. For more details and a few chuckles, see the section:
Laser Humor.

Rangefinders

There are a variety of ways of using lasers to measure distance. The precise
3-D shape or profile of solid objects can be determined using laser scanning
techniques. Common approaches include:

Time of flight - This is what usually comes to mind when this topic comes
up. However, it is probably the most difficult, especially for short to
medium distances where high resolution is desired. Light travels about 1
foot (30.5 cm) per nanosecond (ns). So, the simplistic method of just
measuring the round trip from the reflection of a pulsed laser beam requires
extraordinarily precise timing as the range decreases and the desired
resolution increases. While methods based on phase rather than TOF
(Time-Of-flight) measurement, the practical difficulties are still
formidable. See the section: Comments on
Laser Rangefinders.

Chirped pulse lidar/radar - The laser transmitter sends out an optical
signal which include a subcarrier with a time-varying (chirped) frequency.
If the frequency changes linearly, the difference between the outgoing and
detected signal frequencies is a constant (over the duration of the chirp)
and the distance is then:

Triangulation - Common optical rangefinders such as found in 35 mm cameras
measure distance using the difference in angles of the line-of-sight from an
LED (not even a laser) to the scene and back to a photodetector mounted a
few cm away. As the distance is reduced, the angle increases. This is
coupled to the lens focusing mechanism. The use of a well collimated laser
would increase both the maximum useful distance and resolution of such a
system. See the section: Optical
Rangefinders and Simple Laser Rangefinder
Based on Triangulation. An example of a commercial short range laser
rangefinder based on triangulation techniques can be found at
Aculux Web site. Also see
Webcam
Laser Rangefinder and
University of
Buffalo Robotics - Laser Rangefinder.

Dynamic implementation in the form of a laser scanner can actually be used
to implement a 3-D profile measurement system. If a laser beam is scanned
across a 3-D object, and the spot is viewed (by optical sensors) from two
different locations, it is possible to determine the instantaneous distance
to the spot (on the object). This can be down digitally (using a pair of
CCD cameras - slow) or analog using a pair of 4-quadrant photodiodes. With
a more constrained system (see below), only a single sensor is needed. This
isn't a simple project either but at least doesn't depend on precision on
the order of the wavelength of light! Such scanners exist and are used
in conjunction with robotics (and other research), in industrial CAD/CAM for
construction of computer models from real-world objects, and many other
applications.

(From: Steve Roberts.)

One approach is to use a frame grabber, a translation stage, and a simple
laser with a simple line generating optic. You put the piece to be scanned
on the translation stage, shoot the line onto it from above and look at it
with the camera. The line creates a cross-section of one small part of the
object and the camera records it. Then you process out the laser light from
the background, advance the translation stage one more linewidth, and take
the next slice and so on - sort of a crude from of computed tomography.

(From: Paul Mathews (optoeng@whidbey.com).)

You might want to look at some modules designed for this purpose. The
Sharp's
Distance Measuring Sensors are compact and sensitive. They include the
emitter LED, detector photodiodes, and signal processing circuitry in a
compact integrated module.

There are also some nice application notes available from
Hamamatsu for use with
their Position Sensing Diodes and related ICs.

Interferometry - For very precise measurement of small changes in position,
the use of the wave nature of coherent laser light cannot be surpassed.
Resolution down to a few nm is possible with relatively simple (though
expensive) equipment. See the section: Basics
of Interferometry and Interferometers.

Laser Atlanta Optics is an example
of a company that specializes in laser based distance and speed measureing
technology.

This is the basic principle used in 35 mm rangefinder cameras and other
devices where you view the distance scene and turn a knob to line up two
images that are either superimposed or split top/bottom half. In the case of
the camera, turning the lens focus ring adjusts the angle of mirror A below.

The further apart the mirrors are (size of baseline), the greater the useful
range. Adjust the angle of mirror A or D until the images are
superimposed. Calibrate the angular setting to distance.

The distance from A to the scene is then: tan(angle A) * baseline.

For long distances, C and D can be eliminated - they compensate for the
difference in path lengths of the two views - else the sizes would not be the
same. (Even this doesn't work perfectly in any case. Can you figure out why?)

You can add telescopes and other optics if you like - this is just the basics.

Look Ma, no electronics. :-)

Note that SLR cameras do NOT use this approach as they are entirely optical
(meaning that adjusting the focus only controls the lens - nothing else!).
With SLRs, a pair of shallow prisms oriented in opposite directions (or many
in the case of a 'microscreen' type) are cemented onto a clear area of the
ground glass. When the image is precisely focused onto the ground glass, the
prisms have no effect. However, when the image is in front or behind, they
divert the rays such that the two halves of the image move apart (or the image
breaks up in the case of the 'microscreen').

There were some "Amateur Scientist" articles in Scientific American a
few decades ago on constructing several types of optical range
finders. These were included in the book, "Light and Its Uses". See the
section: A HREF="laserclt.htm#cltsi">Scientific American Articles on Lasers
and Related Topics.

My students construct a simple laser rangefinder using a few basic parts:

Equipment:

HeNe, diode laser or even laser pointer (1/2 mW will do)

Rotary table (to measure second reflected beam angle)

Beamsplitter (a simple glass plate will do)

Optical bench (to put all the optics on)

Flat front-surface mirror (to be mounted on the rotary table)

Basic procedure:

Place the laser to the left of the optical bench. Follow standard safety
procedures for using 1/2 mW lasers.

About 3 inches to the right of the laser aperture (opening), place the beam
splitter at an angle of 45 degrees with respect to (wrt) the incident beam.
This will split the beam into two different paths. Most of the beam will
pass through the splitter. Some will be reflected at a right angle wrt the
incident beam.

About 6 feet to the right of the splitter, place the rotary table with the
mirror on it and face it toward the beam that passes through the splitter.

Now, before you turn on the laser, make sure you have a safe place to aim
the beam for the distance you want to determine.

Now fire up the laser. Note where the first reflected beam strikes the
target (a wall maybe?). Now, slowly and carefully rotate the rotary table
until the beam reflected from the mirror coincides with first reflected
beam. You now have formed a right triangle made of laser light! Pretty
neat! Remember to respect the beam, especially with respect to your eyes!!!

It's not the most precise rangefinder - i.e., the equation is pretty
sensitive to the angular precision of the rotary table. However, it does
demonstrate the basic principle. Maybe the diagram below will help with
setting up the laser rangefinder.

Of course, you can make the non-laser version of this type of rangefinder (but
this is a laser FAQ! --- sam). My students also make that one as well. Both
are pretty neat and demonstrate the power of trig to determine distances!

I am just finishing the development of a range finder based on the TOF
(pulse-Time-Of-Flight) measurement method. There are also different methods
like phase-shift method which compares the phase shift between outgoing
modulated beam and reflected light.

The Pulse TOF method has some advantages which make it very useful: you can
use relatively high pulse power and still be in the Class I safety range.

While building such a range finder there are two crucial components which have
influence on its accuracy: the time measurement circuits and the receiver. Our
aim was to build a laser scanner with the resolution of 1 cm which means that
you have to be able to measure the time with the resolution of 67 ps. The
range of the scanner should be approx. 30m. We are not ready yet but there are
some results.

For the first prototype we used a 1.25 GHz oscillator and special microstrip
design to get the resolution of 70 ps. In the current prototype we use a
special prototype IC which should deliver 50 ps resolution.

The problems are on the receiver side, a relatively large jitter (which I'm
fighting now) destroys my high time measurement precision. The jitter on the
input results in the distance differences of approximately 10 cm). This can be
filtered out by averaging of a number of measurements and that is what we are
doing now. Our measurement frequency is at present 100 kHz, but we will
probably perform the averaging over 10 measurements so that effective
measurement rate will be 10 kHz.

(From: jfd (jezebel@snet.net).)

The problem is getting simultaneous long standoff range and extremely accurate
range. You can phase detect with accuracies in the sub-inch range using direct
detected RF modulated LIDARS or you can use an interferometric technique with a
reference to get sub-micron distances.

(From: Robert (romapa@earthlink.net).)

For much better resolution than would be possible with simple sampling while
still maintaining low cost, digital TOF rangefinders can combine a precision
analog temporal interpolator with say a CMOS system running at 100 MHz.
The analog circuitry to accomplish this is in many production units (for
different applications) - but 5 ps resolution has been achieved with low-cost
components and in production for 15 years from at least one manufacturer.
The idea is interpolate between the digital count periods with a precision
time-to-voltage converter which is then sampled by microcontroller and
combined with the digital counter results.

(From: Bill Sloman (bill_sloman@my-deja.com).)

You may be able to achieve this at low unit cost, but getting a
precision analog temporal interpolator to work well next to CMOS
running at 100 MHz isn't something I'd describe as easy.

We developed a system of this sort at Cambridge Instruments between 1988 and
1991 using a mixture of 100K ECL and GigaBit Logic's GaAs for the digital
logic. Any digital signal going to or from the analog temporal
interpolator was routed as a balanced pair on adjacent tracks, and we
were very careful about the layout, but we still had to work at getting
the noise on the interpolator output down to the 60 picosecond jitter on our
800 MHz master clock (getting a better master clock was the next
priority).

Current-steering logic (like ECL and GaAs) is a lot quieter than
voltage-steering logic (like TTL and CMOS), which is why very fast DACs
and ADCs use ECL interfaces. Precision analog interpolators are no less
sensitive.

Do you know who has actually achieved that 5 ps resolution and for what
application? Tektronix and time domain reflectometers come to mind, though
Tektronix isn't exactly cheap. IIRR Triquint was originally their in-house
analog foundry and I think Tektronix has been using GaAs ASICs in their
faster gear for quite some time now.

The hybrid approach certainly isn't new, but getting it to work is a fair test
of one's analog skills.

Of course, using phase-shift not only makes for easier circuit design,
but also lets you run your LED at a 50% duty cycle, giving you a lot
more reflected photons to work with than the 0.01% you get with TOF.

(From: Lou Boyd (boyd@fairborn.dakotacom.net).)

The Texas Instruments book "Optoelectronics: Theory and Practice" published by
McGraw-Hill had a chapter (23) on the design of an LED/Si Diode rangefinder
with schematics of the transmitter, receiver, and timing section. This was a
phase modulated design but obsolete by todays standards. Low cost modern
rangefinders like those by Leica or even Bushnell are far more advanced in the
detection circuit than that in the TI book. Most eye-safe commercial
rangefinders use phase modulated techniques. This gives good accuracy but
limited range, usually less than 1 kilometer with measurement times typically
1/10 second.

Most military rangefinders use a much higher power transmitter with a time of
flight method. A time of flight rangefinder just sends a single pulse and
receives it. Some use multiple pulses for improved resolution and range but
that typically isn't necessary. A counter is started on the rising edge of
the transmitted pulse and stopped when the rising edge of the receive pulse is
detected. If the counter is measuring a 150 MHz (approx) clock the range will
be displayed in meters. Unfortunately that fast of counter requires at least
a few high speed chips beyond the capability of standard CMOS or TTL logic.
Since the round trip takes only 6.667 microseconds per kilometer you don't
even need blanking on the displays. They can be attached directly to the
counters or just read by a computer. A four or five digit counter suffices
for most purposes. There is a little added complexity on sophisticated units
for making the sensitivity of the receiver increase with time after the pulse
is transmitted. This is sometimes done by charging a capacitor attached to a
gain control which increases the gain with the square of time out to the
maximum the unit is capable of. These rangefinders tend to be expensive
because of the technology but the electronics is simple in concept. Ranges
are limited only by the transmit power which can be extremely high using solid
state Q switched lasers.

Surplus lasers and the associated electronics
from military rangefinders have been showing up on the surplus market in the
$300 range. Unfortunately the receivers have not.

For some insight on
the level of complexity involved look at the boards sold by
E-O Devices These are time of flight
pulsed laser rangefinder components designed for use primarily with LED's or
diode lasers. Also check Analog
Modules for examples of state of the art variable gain rangefinder
receivers. If you want one of their modules plan on spending between $1,000
and $2,000. :-(

Phase shift methods allow achieving high precision in distance
resolution with lower power and lower speed circuitry. That equates to
lower cost and higher precision. Which type is best depends on what
properties are needed.

Single pulse rangefinders typically use YAG or erbium lasers while most
of the phase shift type use diode lasers.

(From: Don Stauffer

Which type to use depends a bit on what range resolution you are looking
for. If you want high resolution, you will be working with a high
modulation frequency. Then you may find many circuits designed for
receiving audio modulation may not provide enough bandwidth.

Also, there is the range ambiguity problem. If you go high enough in
frequency, you may find some range ambiguity.

You will also likely be needing very accurate phase measurement circuits
if you are using moderate modulation frequency, so study carefully high
accuracy phase detectors. These are not trivial circuits. In order for
them to work well, you need a pretty good SNR.

(From: A. E. Siegman (siegman@stanford.edu).)

Adding to what others have said, hand-held laser rangefinders using
low-power RF-modulated CW lasers (a.k.a. diode lasers) together with
phase-detection techniques are simpler, cheaper, smaller, *much* more
battery efficient, and much safer; and are more or less replacing the
pulsed hand-held versions of yore.

These techniques are also moderately old. Coherent (maybe Spectra also) were
making widely used laser surveying instruments ("Geodolite"?) that
worked this way a couple decades or more ago (and there may have
been incoherent light source versions even further back).

I suppose that compared to TOF, one disadvantage is that it takes longer to
integrate up the signal to get a range finding, and if you're in a tank
battle and want to get off the first shot before alerting the enemy that
you're illuminating him and giving him a chance to duck, the pulsed type may
still be better.

Do some web searching: You can buy binoculars with a built-in diode
laser rangefinder from Amazon, and use it to measure the distance to the
pin on your next golf outing.

(From: Louis Boyd (boyd@apt0.sao.arizona.edu).)

Prior to laser diodes (1960's) there were optical geodimeters which
used a tungsten lamp, a Kerr shutter (which modulates light at
multi-megahertz rates using polarizers and high voltage rf driven
nitrobenzene), and photomultiplier receivers. These could measure
distances to a few centimeters at ranges of several kilometers. They
were large, expensive, and a bi*ch to calibrate. They used phase shift
techniques similar to modern diode rangefinders, but without the aid of
microprocessors. They switched modulation frequencies to resolve phase
ambiguities.

Modern rangefinders often use pseudorandom modulation and
cross-correlation computation to give the round-trip delay which is
proportional to distance. Distance resolution can be much finer than
the length of the shortest pulse.

With modern geodimeters the distance accuracy is primarily limited by
uncertainty of light propagation velocity in the air since it's not
practical to measure the pressure and humidity at all points along the
path, but can be accurate to better than 1 part in 10^6 with care. Tape
and chain is difficult to get better than 1 part in 10^3 which is the
typical accuracy of $200 pocket laser rangefinders.

(From: Mike Poulton (mpoulton@mtptech.com).)

Using pulses is not very practicable - if you want to achieve a resolution of
a few mm over a distance of 100 m or so, you find that you'd need extremely
short pulses (recall that 1 ns corresponds to 30 cm or 12 inches,
approximately, so you's need pulses of a few ps); you could do this with
a W-switched SS laser, but those little hand-held devices, who do
have a resolution in this order of magnitude, cannot work in this
way. They use a RF-modulated CW signal from a laser diode, say
with 100 MHz, and measure the phase shift of the 100 MHz signal between
outgoing and incoming beams. This phase shift can be very accurately
measured by first converting the 100 MHz down to a few 100 kHz (like
a superheterodyne receiver).

Some while ago I had been interested in such a circuit myself (for
measuring optical path lengths) but didn't find anything useful on the web.

(From: Repeating Rifle (SalmonEgg@sbcglobal.net).)

Equipment of this ilk is called *distance measuring equipment* or DME and
has all but replaced the use of chains in surveying practice. Various
implementations have been used. Some use high frequencies to obtain
precision and lower frequencies for range ambiguity resolution. Others use
inconmensurate frequencies that are not all that different from one another.
I you match the filtering to the transmission, you pretty much get the same
signal to noise ration for all kinds of devices. The broad-band pulses
mentioned above use short pulses. The CW devices use narrow band filters.

The first items of this nature used RF directly without light.

Trade names that come to mind quickly are tellurometer and geodimeter.

For the military rangefinders that use high power pulses, signal processing
is less than optimum. An error of 5 meters will usually not be a big deal.
For surveying, that kind of error will usually be unacceptable. In both
cases extended (in range) targets will introduce error.

Almost all of the inexpensive hand-held rangefinders on the market use a
simplified form of phase detection with relatively low modulation rates.
Phase sensing rangefinders uses a variable pulse width modulated laser diode.
It would use use thousands of on/off transitions in determining each distance
measurement by comparing the modulation pattern to the returned signal using
cross-correlation techniques. Resolution is a function of measurement
time, speed and size of the registers, and instrument stability. Single
pulse TOF rangefinders on the other hand are generally used for very
long ranges (several km and up) with very high pulse power (kilowatts to
megawatts peak) and range resolution rarely better than a meter. Low
power single pulse rangefinders are rare as the expense of the detection
circuits isn't justified for the low resolution.

The accuracy of quality surveying distance meters is limited primarily
by the uncertainty of the velocity of propagation of light through the
atmosphere. That varies of with air pressure and humidity which can't
easily be determined over the entire path. Still, they're orders of
magnitude better than a tape or chain.

(From: Phil Hobbs (pcdh@us.ibm.com).)

Modulated CW measurements also allow you to use very narrow measurement
bandwidths very easily (e.g. with a PLL), which helps the SNR very much. In
shorter range units, sinusoidal modulation can also be used to prevent
back-reflections from causing mode hopping. You choose delta-f so that the
phase modulation of the back-reflection (in radians) is at a null of the
zero-order Bessel function J0. This can make a huge difference (3 orders of
magnitude) in the back-reflection sensitivity.

The following is what I would suggest for a relatively low cost approach
achieving 15 to 50 cm resolution and 100 meter or more range. However, also
see the next section for a much simpler approach that may be adequate.

A Q-switched solid state laser will give you short pulses with minimal fuss.
A unit like the small surplus Nd:YAG laser (SSY1) described in chapter:
Solid State Lasers was originally part of the M-1
tank rangefinders and thus should be ideal. It is quite trivial to build a
suitable power supply these laser heads since a passive Q-switch is used and
this doesn't require any electrical control.

A few mJ should be sufficient. (SSY1 is probably in the 10 to 30 mJ range
using the recommended pulse forming network.) With a Q-switched laser, the
required short pulse if created automagically eliminating much of the
complexity of the laser itself.

Diode laser assemblies from the Chieftain tank rangefinder are also available
on the surplus market but you probably would have to build a pulsed driver for
them which would be more work.

For the detector, a PIN photodiode or avalanche photodiode (APD) would be
suitable. The preamp is the critical component to get the required ns
response time. You need to sample both the pulse going out and the return
since the delay from firing the flashlamp (if you are using a solid state
laser) to its output pulse is not known or constant.

15 cm resolution requires a time resolution of about 1 ns (twice what you
might think because the pulse goes out and back). GHz class counters are no
big deal these days.

However, approaches that are partially analog (ramp and A/D) which don't
require such high speed counters are also possible. In fact, if your digital
design skills aren't so great, this is probably the easiest way to get decent
resolution, if possibly not the greatest accuracy/consistency. All you need
is a constant current source and an A/D (Analog to Digital converter). This
can be as simple as a FF driving a transistor buffer to turn the voltage to
charge the capacitor on and off with a transistor set up with emitter feedback
for as a constant current source. Or, it can just be an exponential charge
with non-linear correction done in software. The A/D doesn't need to be fast
as long as its output word has enough bits for your desired resolution. For
a typical exponential charging waveform, add 1 bit to the required A/D word
size. For example, determining distance over 100 meters to to 5 cm resolution
would require that the full voltage ramp be about 700 ns in duration (a bit
over maximum round trip time, cut off sooner if there is a return pulse) and
then sampled with a 12 bit A/D.

Another even simpler way of doing this is to charge the capacitor as above
but then discharge it with a much longer time constant and determine how long
it takes to reach a fixed voltage. By making the discharge time constant
sufficiently large, any vanilla flavored microprocessor could be used for
control and timing.

All in all, these are non-trivial but doable projects.

See the previous sections on laser rangefinders for more info.

Here is a Web site that appears to go into some detail on the design of
TOF laser rangefinders:

A laser phase shift distance meter can be constructed by analog
modulation of the laser and measuring the phase shift of the return
signal. With some filtering you can do multiple frequencies at the
same time. Also, the feedback diode in a semiconductor laser can be
used as the sensor (in which case the circuitry gets interesting).
High precision can be accomplished relatively easily. I'm trying to
get better than 0.1 mm (preferably better than 0.01 mm) over short
distances (a couple of meters).

This is a slightly modified approach and may be made to work with relatively
simple inexpensive circuitry. The idea is to use a normal IR or visible
laser diode (e.g., such as from a CD or DVD player) in conjunction with a
common photodiode to form an oscillator whose frequency will depend on the path
delay between them - i.e., the distance to the "target". Basically, the
laser diode is turned on which sends out a leading edge of a light pulse.
The light hits the target and is reflected back into the photodiode, which
turns the laser diode off. The loss of signal then turns the laser diode
on and the cycle repeats continuously. The oscillating frequency is
then equal to 1 over (4 times the distance to the target plus 2 times
the internal circuit delay). A simple frequency to voltage converter
drives an analog meter. No really high speed components are needed.

I'm not convinced that the circuit as presented works - there is at least
one part value (C4, 100 uF) which would appear to be much larger than
desired inside the feedback loop. The principle appears valid though.

Each pixel of a CCD-based image sensor accumulates charge proportional to the
light intensity and shutter open or "gate time". For normal video, the
electronic shutter is open for a duration which is a large fraction of a video
frame to maximize sensitivity and minimize aliasing in moving images. For stop
motion photography, much shorter shutter open times are used. If it were
possible to synchronize the electronic shutter with the generation of a light
pulse illuminating the scene, then the amount of charge in each CCD cell would
also depend on how long it takes for the light to reach the CCD (since the
shutter would close before the light from more distant points returned). One
problem, of course, is that this is possible only under very special
conditions. A way to get around this would be to do the measurement in two
steps:

First, an intensity image reference is captured where the shutter is open
long enough so that distance isn't a factor. The light source can be pulsed
with a long shutter time or the light source could be on for a time
corresponding to at least twice the round-trip time at maximum distance
with a shutter open for the duration of the round trip time.

Next, the light source is pulsed for a length of time equal to the
round-trip time based for the maximum distance of interest and electronic
shutter is gated with the same pulse. The accumulated charge would then be
inversely related to distance and proportional to intensity. But the
intensity contribution could be subtracted out since it is known from the
reference frame.

In order for this to be implemented with a normal CCD camera, either direct
control of the electronic shutter is needed, bypassing any synchronous logic,
or a "sync" output from the camera must be available. Also note that the
charge integration times involved - 10s or 100s of ns - are orders of
magnitude smaller than those normally used on all but very specialized CCD
cameras, even with a fast shutter. So, sensitivity is going to be very low.
A high power pulsed laser may be needed to generate adequate photons and even
then, the CCD may not be able to supply enough charge.

The simplist way of doing this may be to use the existing focusing mechanism of
the pickup. Focus in a CD or DVD device depends on a reflection from a
relatively flat smooth surface (the metalized information layer of the disc/k)
to produce an elliptical spot back at the photodiode array. The major axis of
the ellipse lies on a diagonal (45 or 135 degrees) and depends on the distance
above or below optimal focus - at that point, it is a perfect circle. A four
quadrant photodetector takes the difference of the amplitude of the return
signals from the two pairs of diagonally opposed quadrants to determine the
focus error. See the document:
Notes on the
Troubleshooting and Repair of Compact Disc Players and CDROM Drives for
more on how optical pickups actually work.

If the surface is smooth and flat over a scale of 5 to 10 um, this could work
as a way of determining distance to the pickup. In other words, the dominant
return from the surface has to be a specular reflection back to the source in
order for the focus servo to lock properly. (The width and depth of the
pits/lands of the CD or DVD disc is small compared to the beam so they are
mostly ignored by the focus servo.) I don't know how much angular deviation
could be tolerated.

The output would be an analog voltage roughly proportional to focus error
which could be mapped to lens height (assuming the device is in a fixed
orientation with respect to gravity - more complex if you want to do this
while on a roller coaster or in microgravity!). The total range would be 1 to
2 mm with an accuracy of a few um.

Conventional optical encoders - whether they are the dirt-cheap variety inside
your computer mouse or the precision type found in industrial robots and other
machine tools - consist of a light source or sources, some means of
interrupting or varying the light intensity based on linear position or
rotation angle, and photodetectors to convert the light to an electrical
signals. By using various patterns on film or glass strips or discs, relative
(2 bits) or absolute (many bits) measurements can be made with a computer or
dedicated logic calculating position or angle, speed or rotation rate,
acceleration, and so forth from this data. Through clever design and
careful manufacturing, extremely high resolution is possible using conventional
LEDs or incandescent lamps for the light source(s). However, lasers can be
used as well with some potential advantages - even higher precision and
stand-off (some distance between the moving parts) operation.

Since the 'stylus' of a CD player has an effective size of around 1 um (DVD
would be even less), it could in principle be used to implement a very high
resolution optical encoder for use in linear, rotary, or other sensing
application. The stand-off distance (from objective lens to focal point) can
be a couple of mm which may be an advantage as well. While this is probably
somewhat less difficult than turning a CD player into an interferometer (see
below), it still is far from trivial. You will have to create an encoder disc
or strip with a suitable reflective pattern with microscopic dimensions.
Without access to something like a CD/DVD mastering unit or semiconductor
wafer fab, this may be next to impossible. Your servo systems will need to
maintain focus (at least, possibly some sort of tracking as well) to the
precision of the pattern's feature size. To obtain direction information,
the 'track' would need to have a gray code pattern similar to that of a normal
optical encoder - but laid down with um accuracy in such a way that the
photodiode array output would pick it up. (Implementing an absolute encoding
scheme would probably require so many changes to the pickup as to make it
extremely unlikely to be worth the effort.) Of course, you also need laser
diode driver circuitry and the front-end electronics to extract the data
signal. Not to mention the need for a suitable enclosure to prevent
contamination (like lathe turnings) from gumming up the works. And, with your
device in operation, any sort of vibration or mechanical shock could cause a
momentarily or longer term loss of focus and thus loss of your position or
angle reference.

Speed is just the rate of change of position so any of the approaches that
measure position can be adapted for speed measurements by simply taking
a pair of readings and computing their difference with respect to time.
More direct methods using CW lasers depend on using some form of the doppler
shift of the reflected beam, usually of a subcarrier imposed on the the
laser beam by amplitude modulation.

For example, if the outgoing laser beam is modulated at 1 GHz and the
reflected beam is combined with this same reference 1 GHz in the sensor
photodiode or a mixer, for relative speeds small compared to c (the velocity
of light), the difference frequency will be approximately 1 Hz per 0.5
foot/second.

As an example of an interferometer for making precise physical measurements,
split a beam of monochromatic coherent light from a laser into two parts,
bounce the beams around a bit and then recombine them at a screen, optical
viewer, or sensor array. The beams will constructively or destructively
interfere with each-other on a point-by-point basis depending on the net
path-length difference between them. This will result in a pattern of light
and dark fringes. If one of the beams is reflected from a mirror or corner
reflector mounted on something whose position you need to monitor extremely
precisely (like a multi-axis machine tool), then as it moves, the pattern will
change. Counting the passage of the fringes can provide measurements accurate
to a few nanometers!

The laser produces a coherent monochromatic beam which is expanded and
collimated by a pair of positive lenses (not shown).

Part of the laser beam is reflected up by the Beamsplitter (half silvered
mirror), reflects off of Mirror 1 and back down. A portion of this passes
through the Beamsplitter to the Screen.

The remainder of the laser beam passes through the Beamsplitter and is
reflected from Mirror 2. Part of this is reflected down by the Beam
Splitter to the Screen.

The two beams combine at the Screen resulting in an interference pattern
of light and dark fringes or a full field varying between light and dark
as the path length is changed. A magnifier, microscope, or other optical
system imaging to a human observer or electronic sensor may be provided in
place of the screen to view the fringe pattern in more detail or provide
input to an electronic measurement system.

In a perfectly symmetric Michelson interferometer, the fringe pattern should
uniformly vary between bright and dark (rather than stripes or concentric
circles of light) depending on the phase difference between the two beams
that return from the two arms. A circular pattern is expected if the two
curvatures of the wavefront are not identical due to a difference in
arm-lengths or differently curved optics. Stripes (straight or curved) in
any direction) would be an indication of a misalignment of some part of the
interferometer (i.e. the beams do not perfectly overlap or one is tilted
with respect to the other).

A microscopic shift in position or orientation of either mirror will result
in a change to the pattern. Presumably, the mirror designated as 'Moving'
is mounted on some equipment such as a disk drive head positioner that is
being tested or calibrated. For these applications, setting up the
interferometer is set up to produce a fringe pattern with at least two
sensors to determine direction and velocity in a sophisticated version of
the A-B quadrature decoder used in your typical computer mouse. :)

(Yes, about 50 percent of the light gets reflected back toward the laser and
is wasted with this particular configuration. This light may also destabilize
laser action if it enters the resonator. Both of these problems can be easily
dealt with using slightly different optics than what are shown.)

A long coherence length laser producing a TEM00 beam is generally used for
this application. HeNe lasers have excellent beam characteristics especially
when frequency stabilized to operate in a single longitudinal mode. However,
some types of diode lasers (which are normally not thought of as having
respectable coherence lengths or stability) may also work. See the section:
Interferometers Using Inexpensive Laser
Diodes. Even conventional light sources (e.g., gas discharge lamps
producing distinct emission lines with narrow band optical filters) have
acceptable performance for some types of interferometry.

Such a setup is exceedingly sensitive to EVERYTHING since positional shifts
of a small fraction of a wavelength of the laser light (10s of nm - that's
nanometers!) will result in a noticeable change in the fringe pattern. This
can be used to advantage in making extremely precise position or speed
measurements. However, it also means that setting up such an instrument in a
stable manner requires great care and isolated mountings. Walking across the
room or a bus going by down the street will show up as a fringe shift!

Interferometry techniques can be used to measure vibrational modes of solid
bodies, the quality (shape, flattness, etc.) of optical surfaces, shifts in
ground position or tilt which may signal the precursor to an earthquake, long
term continental drift, shift in position of large suspended masses in the
search for gravitational waves, and much much more. Very long base-line
interferometry can even be applied at cosmic distances (with radio telescopes
a continent or even an earth orbit diameter apart, and using radio emitting
stars or galaxies instead of lasers). And, holography is just a variation on
this technique where the interference pattern (the hologram) stores complex
3-D information.

This isn't something that can be explained in a couple of paragraphs. You
need to find a good book on optics or lasers. Here are some suggestions
for further study:

Gordon McComb's: "The Laser Cookbook [1} and the Scientific American
collection: "Light and its Uses [5]" include various type of interferometers
which can be built with (relatively) readily available parts.

Agilent (among others) manufacture
'Laser Interferometry Measurement Systems' based on these techniques.
Information and application notes are available by searching
for the key words: "Laser" or "Dimensional Measurement". For Agilent in
particular, searching for "5501" or "5517" will find information on their
specific systems.

The Astroweb Internet
Resources for Astronomy Web site (and others). There are links to
people interested in designing, building, and operating various types of laser
interferometers. Much of the information relates directly to the testing
of optical components for astronomical telescopes but there should be much of
general interest as well.

Suppose we have a Michelson interferometer (see the section:
Basics of Interferometry and
Interferometers) set up with a perfectly collimated (plane wave source)
and perfectly plane mirrors adjusted so that they are perfectly perpendicular
to the optical axis (for each mirror) and the beamsplitter is also of perfect
construction and oriented perfectly. In this case, there won't be multiple
fringes but just a broad area whose intensity will be determined by the
path-length difference between the two beams. Where this is exactly 1/2
wavelength (180 degrees), the result will be nothing at all and the screen
will be absolutely dark! So, where is all the energy going? No, it doesn't
simply vanish into thin air or the ether, vacuum, the local dump, or anywhere
else. :-)

Your initial response might be: "Well, no system is ideal and the beams won't
really be perfectly planar so, perhaps the energy will appear around the
edges or this situation simply cannot exist - period". Sorry, this would be
incorrect. The behavior will still be true for the ideal case of perfect
non-diverging plane wave beams with perfect optics.

Perhaps, it is easier to think of this in terms of an RF or microwave,
acoustic, or other source:

What would happen if a continuous wave signal were split into two parts and
then recombined 180 degrees out of phase? Or a sinusoidal signal from a
quartz oscillator split into two equal parts on a pair of transmission lines
(coax or whatever) and recombined 180 degrees out of phase? Less mysterious?

Assuming energy doesn't actually disappear (it doesn't), what else must be
going on to account for a null at the point where they combine? What does
each signal see? How is it affected?

What about with just an acoustic resonance inside an organ pipe or even a
low-tech standing wave on a piece of string? Oops, Did I say resonance and
standing waves? :-)

Hint: From the perspective of either of the two signals, how is this different
(if at all) than imposing a node (fixed point) on a transmission line? Or at
the screen of the interferometer? After all, a nodal point is just an
enforced location where the intensity of the signal MUST be 0 but here it is
already exactly 0. For the organ pipe, such a nodal point is a closed end;
for the string, just an eye-hook or a pair of fingers!

OK, I know the anticipation is unbearable at this point. The answer is that
the light is reflected back to the source (the laser) and the entire optical
path of the interferometer acts like a high-Q resonator in which the energy
can build up as a standing wave. Light energy is being pumped into the
resonator and has nowhere to go. In practice, unavoidable imperfections of
the entire system aside, the reflected light can result in laser instability
and possibly even damage to the laser itself. So, there is at least a chance
that such an experiment could lead to smoke!

(From: Art Kotz (alkotz@mmm.com).)

We don't have to to think all that hard to figure out where all the energy
is dissipated in a Michelson interferometer. Nor do we have to refer to
imperfect components either. The thought experiment of perfect non-absorbing
components still renders a physically correct solution.

To summarize a (correct) previous statement, in a Michelson interferometer
with flat surfaces, you can get a uniform dark transmissive exit beam. The
power is not dissipated as heat. There is an alternate path that light can
follow, and in this case, it exits the way it came in (reflected back out to
the light source).

In fact, with a good flat Fabry-Perot interferometer, you can actually
observe this (transmission and reflection from the interferometer alternate
as you scan mirror spacing).

In the electrical case, imagine a transmitter with the antenna improperly
sized so that most of the energy is not emitted. It is reflected back to the
output stage of the transmitter. If the transmitter can't handle dissipating
all that energy, then it will go up in smoke. Any Ham radio operators out
there should be familiar with this.

(From: Don Stauffer (stauffer@htc.honeywell.com).)

Many of the devices mentioned have been at least in part optical resonators.
It may be instructive to look at what happens in an acoustic resonator like an
organ pipe or a Helmholtz resonator.

Let's start with a source of sound inside a perfect, infinite Q resonator.
The energy density begins to build up with a value directly proportional to
time. So we can store, theoretically, an infinite amount of acoustic energy
within the resonator.

Of course, it is impossible to build an infinite Q resonator, but bear with me
a little longer. It is hard to get an audio sound source inside the resonator
without hurting the Q of the resonator. So lets cut a little hole in the
resonator so we can beam acoustic energy in. Guess what, even theoretically,
this hole prevents the resonator from being perfect. It WILL resonate.

No optical resonator can be perfect. Just like in nature there IS no
perfectly reflecting surface (FTIR is about the closest thing we have). Every
time an EM wave impinges on any real surface, energy is lost to heat. With
any source of light beamed at any surface, light will be turned into heat. In
fact, MOST of the energy is immediately turned to heat. By the laws of
thermodynamics, even that that is not converted instantaneously into heat, but
goes into some other form of energy, will eventually turn up as heat. You pay
now, or you pay later, but you always pay the entropy tax.

(From: Bill Vareka (billv@srsys.com).)

And, something else to ponder:

If you combine light in a beamsplitter there is a unavoidable phase relation
between the light leaving one port and the light leaving the other.

So, if you have a perfect Mach-Zehnder interferometer like the following

If you set it up so that there is total cancellation out of, say, port A, then
Port B will have constructive interference and the intensity coming out port B
will equal the combined intensity coming in the two input ports of that final
beamsplitter. This is due to the phase relation between the light which is
reflected at the beamsplitter. That which is reflected and goes out port A
will be 180 degrees out of phase with that which is reflected and goes out
port B. The transmitted part of port A and port B are the same. Hence the
strict phase relationship between the light from the two output ports. This
is an unavoidable result of the time-reversal symmetry of the propagation of
light.

(From: A. Nowatzyk (agn@acm.org).)

A beam-splitter (say a half silvered mirror) is fundamentally a 4 port device.
Say you direct the laser at a 45 degree angle at an ideal, 50% transparent
mirror. Half of the light passes through straight, the rest is reflected at a
90 degree angle. However, the same would happen if you beam the light from
the other side, which is the other input port here. If you reverse the
direction of light (as long as you stay within the bounds of linear optics,
the direction of light can always be reversed), you will see that light
entering either output branch will come out 50/50 on the two input ports. An
optical beam-splitter is the same as a directional coupler in the RF or
microwave realm. Upon close inspection, you will find that the two beams of a
beam-splitter are actually 90deg. out of phase, just like in an 1:1
directional RF coupler.

In an experiment where you split a laser beam in two with one splitter and
then combine the two beams with another splitter, all light will either come
out from one of the two ports of the second splitter, depending on the
phase. It is called a Mach-Zehnder interferometer.

Ideal beam-splitters do not absorb any energy, whatever light enters will come
out one of the two output ports.

We all know that light from a single coherent source can create interference
patterns and such. What about arbitrary uncorrelated sources?

There will be interference but you won't see any visible patterns unless the
two sources are phase locked to each-other since even the tiny differences in
wavelength between supposedly identical lasers (HeNe, for example) translate
into beat frequencies of MHz or GHz!

(From: Charles Bloom (cbloom@caltech.edu).)

The short answer is yes.

Let's just do the math. For a wave-number k (2pi over wavelength), ordinary
interference from two point-like apertures goes like:

This is almost a nice interference pattern as we vary 'a', but we've got some
nasty L dependence, and in the regime L >> a where our approximations are
valid, the L dependence will dominate the a dependence (unless (k-K) is very
small; in particular, we'll get interference roughly when a(k+K) ~ 10 and
L(k-K) ~ 1 , and L >> a , which implies |k-K| << |k+K| , nearly equal
wavelengths.)

The L dependence is the usual phenomenon of "beats" which is also a type of
interference, but not the nice "fringes" we get with equal wavelengths (the L
dependence is like a Michelson-Morely experiment to compare wavelengths of
light, by varying L (the distance between the screen and the sources) I can
count the frequency of light and dark flashes to determine k-K.

Building something that demonstrates the principles of interferometry may not
be all *that* difficult (see the comments below). However, constructing a
useful interferometer based measurement system is likely to be another matter.

So you would like to add a precision measurement system to that CNC machining
center you picked up at a garage sale or rewrite the servo tracks on all your
dead hard drives. :) If you have looked at Agilent's products - megabucks
(well 10s of K dollars at least), it isn't surprising that doing this may be
a bit of a challenge. As noted in the section:
Basics
of Interferometry and Interferometers, a high quality (and expensive)
frequency stabilized single mode HeNe laser is often used. For home use
without one of these, a short HeNe laser with a short random polarized tube
(e.g., 5 or 6 inches) will probably be better than a high power long one
because it's possible only 2 longitudinal modes will be active and they will
be orthogonally polarized with stable orientation fixed by the slight
birefringence in the mirror coatings. As the tube heats up, the polarization
will go back and forth between the two orientations but should remain constant
for a fair amount of time after the tube warms up and stabilizes. Also see
the section: Inexpensive Home-Built Frequency
or Intensity Stabilized HeNe Laser.

The problem with cheap laser diodes is that most have a coherence length that
is in the few mm range - not the several cm or meters needed for many
applications (but see the section: Can I Use
the Pickup from a CD Player or CDROM Drive for Interferometry?). There
may be exceptions (see the section:
Interferometers Using Inexpensive
Laser Diodes) and apparently the newer shorter wavelength (e.g., 640 to
650 nm) laser pointers are much better than the older ones but I don't know
that you can count on finding inexpensive long coherence length laser diodes.
Even if you find that a common laser diode has adequate beam quality when you
test it, the required stability with changes in temperature and use isn't
likely to be there.

The detectors, front-end electronics, and processing, needed for an
interferometer based measurement system are non-trivial but aren't likely to
be the major stumbling block both technically and with respect to cost. But
the laser, optics, and mounts could easily drive your cost way up. And,
while it may be possible to use that $10 HeNe laser tube, by the time you
get done stabilizing it, the effort and expense may be considerable.

Note that bits and pieces of commercial interferometric measurings systems
like those from HP do show up on eBay and other auction sites from time to
time as well as from laser surplus dealers. The average selling prices are
far below original list but complete guaranteed functional systems or rare.

(From: Randy Johnson (randyj@nwlink.com).)

I'm an amateur telescope maker and optician and interferometry is a technique
and method that can be used to quantify error in the quality of a wavefront.
The methods used vary but essentially the task becomes one of reflecting a
monochromatic light source, (one that is supplied from narrow spectral band
source i.e., laser light) off of, or transmitting the light through a reference
element, having the reference wavefront meet the wavefront from the test
element and then observing the interference pattern (fringes) that are formed.
Nice straight, unwavering fringe patterns indicate a matched surface quality,
curved patterns indicate a variation from the reference element. By plotting
the variation and feeding the plot into wavefront analysis software (i.e., E -Z
Fringe by Peter Ceravolo and Doug George), one can assign a wavefront rating
to the optic under test.

The simplest interference test would involve two similar optical surfaces in
contact with each other, shining a monocromatic light source off the two and
observing the faint fringe pattern that forms. This is known as a Newton
contact interferometer and the fringe pattern that forms is known as Newton's
rings or Newton's fringes, named for its discoverer, you guessed it, Sir Issac
Newton. If you would like to demonstrate the principle for yourself, try a
couple of pieces of ordinary plate glass in contact with each other, placed
under a fluorescent light. Though not perfectly monochromatic, if you observe
carefully you should be able to observe a fringe pattern.

Non-contact interferometry is much tougher as it involves the need to get a
concentrated amount of monochromatic light through or reflected off of the
reference, positioning it so it can be reflected off of the test piece, and
then positioning the eye or imaging device so that the fringe pattern can be
observed, all this while remaining perfectly still, for the slightest
vibration will render the fringe pattern useless.

(From: Bill Sloman (sloman@sci.kun.nl).)

An interferometer is a high precision and expensive beast ($50,000?). You use
a carefully stabilized mono-mode laser to launch a beam of light into a cavity
defined by a fixed beamsplitter and a moving mirror. As the length of the
cavity changes, the round-trip length changes from an integral number of
wavelengths of light - giving you constructive interference and plenty of
light - to a half integral number of wavelengths - giving you destructive
interference and no light.

This fluctuation in your light output is the measured signal. Practical
systems produce two frequency-modulated outputs in quadrature, and let you
resolve the length of a cavity to about 10 nm while the length is changing at
a couple of meters per second. The precision is high enough that you have to
correct for the changes in speed of light in air caused by the changes
temperature and pressure in an air-conditioned laboratory.

Hewlett-Packard invented the modern interferometer. When I was last involved
with interferometers, Zygo was busy trying to grab a chunk of the market from
them with what looked liked a technically superior product. Both manufacturers
offered good applications literature.

(From: Mark Kinsler (kinsler@froggy.frognet.net).)

You can get interferometer kits from several scientific supply houses. They
are not theoretically difficult to build since they consist mostly of about
five mirrors and a lens or two. But it's not so easy to get them to work
right since they measure distances in terms of wavelengths of light, and
that's *real* sensitive. You can't just build one on a table and have it work
right. One possible source is: Central Scientific Company.

(From: Bill Wainwright (billmw@isomedia.com).)

Yes, you can build one on a table top. I have done it. I was told it could
not be done but tried it anyway. The info I read said you should have an
isolation table to get rid of vibrations I did not, and even used modeling
clay to hold the mirrors. The main problem I had was that the image was very
dark and I think I will use a beamsplitter in place of one of the mirrors
next time. The setup I had was so sensitive that lightly placing your finger
on the table top would make the fringes just fly. To be accurate you need to
take into account barometric presure and humidity.

The party line has tended to be that the coherence length of diode lasers is
too short for interferometry or holography. (See the sections beginning with:
General Interferometers.) While I was aware
of CD laser optics being used with varying degrees of success for relatively
short range interferometry (a few mm or cm - see the section:
Can I Use the Pickup from a CD Player or CDROM
Drive for Interferometry?), the comments below are the first I have seen
to suggest that performance using some common laser diodes may be at least on
par with that of a system based on a typical HeNe laser (though not a high
quality and expensive frequency stabilized single mode HeNe laser).

While I don't know how to select a laser diode to guarantee an adequate
coherence length, it certainly must be a single spatial (transverse) mode
type which is usually the case for lower power diodes but those above 50
to 100 mW are generally multimode. So, forget about trying to using a 1 W
laser diode of any wavelength for interferometry or holography. However,
single spatial mode doesn't guarantee that the diode operates with a single
longitudinal mode or has the needed stability for these applications. And,
any particular diode may operate with the desired mode structure only over
a range of current/output power and/or when maintained within a particular
temperature range.

(From: Steve Rogers (scrogers@pacbell.net).)

I have been involved with laser diodes for the last 15 years or so. My first
was a pulsed (only ones available at that time) monster that peaked 35 watts
at 2 kHz with 40 A pulses! It was a happy day when they could operate CW and
visible to say the least. Anyway, in the course of my working travels, I have
built numerous Twymann-Green double pass interferometers for the wave front
distortion analysis of laser rods, i.e., Nd:Yag, Ruby, Alexandrite, etc. The
standard reference light source for this instrument has always been the 632.8
nm HeNe laser. Good coherence length and relatively stable frequency was its
strong suit.

When visible diode lasers came out I often wondered aloud about their
suitability as a replacement for the HeNe. I despise HeNe lasers. They are
bulky and I have been shocked too many times from their power supplies.

I assumed that since CD player laser diodes at 780 nm could have coherence
lengths on the order of tens of centimeters or into the meters (!!, see, for
example: Katherine Creath, "Interferometric Investigation of a Diode Laser
Source", Applied Optics (24 1-May-1985) pp. 1291-1293), Visible Laser Diodes
(VLDs) could make excellent replacements. As it turned out, VLDs tend to have
coherence lengths which are considerably shorter according to the latest
technical literature and I held off on experimenting with them. Last week, I
went through my shop and found enough mirrors, beamsplitter, assorted optics
to throw together my own double-pass interferometer for home use. This
coincided with my acquisition of a 635 nm 5 mw diode module - a good one from
Laserex.

To make a longer story shorter, I assembled said equipment with the VLD and
WOW! excellent fringe contrast (a test cavity of four inches using a .250" x
4.0" Nd:Yag rod as the test sample.) When a HeNe laser was substituted for
the VLD, virtually no difference in the manual calculation of wave front
distortion (WFD) and fringe curvature/fringe spacing. The only drawback with
the VLD is that it produces a rectangular output beam. When collimated you
have a LARGE rectangular beam rather than a nice round HeNe style beam. My
interferometer now occupies a space of 10" x 10" and is fully self contained.
It probably could even be made smaller. Not only that, but it runs on less
than 3 V!!!

I am just as surprised as you are with the results that I achieved. This is
one reason why it took me so long to attempt this experiment (something like
4 to 5 years). I have always assumed that a HeNe laser would be FAR superior
in this configuration than a VLD would be. Perhaps others may know more about
the physics than I do. One thing is certain, these are "single mode" index
guided laser diodes and typically exhibit the classic gaussian intensity
distribution which is not so evident with the "gain guided" diodes. This in
turn implies a predominant lasing mode which in turn would imply a (somewhat)
stable frequency output. Purists would note that this VLD has a nominal
wavelength of 635 nm +/- 10 nm while the HeNe laser is pretty much fixed at
632.8 nm. This variable could account for extremely minor WFD differences.

(From: W. Letendre (wjlservo@my-dejanews.com).)

There's an outfit in Israel selling a diode based laser interferometer enough
cheaper than Zeeman split HeNe units to suggest that they are using a laser
diode in the 'CD player' class, or perhaps a little better. They are able to
measure, 'single pass' (retro rather than plane mirror) over lengths of up to
about 0.5 m, suggesting that as an upper limit for coherence length.

With the nice precision optics, electromechanical actuators, laser diode, and
photodiode array present in the mass produced pickup of a CD/DVD player,
CD/DVDROM drive, or other optical disc/k drive, one would think that
alternative uses could be found for this assembly after it has served for many
years performing its intended functions - or perhaps, much earlier, depending
on your relative priorities. :-) (Also see the section:
Using a CD or DVD Optical Pickup in a Precision
Position or Angle Encoder.

People sometimes ask about using the focused laser beam for for scanning or
interferometry. This requires among other things convincing the logic in
the CD/DVD player or CD/DVDROM drive to turn the laser on and leave it on
despite the possible inability to focus, track, or read data. The alternative
is to remove the optical pickup entirely and drive it externally.

If you keep the pickup installed in the CD player (or other equipment),
what you want to do isn't going to be easy since the microcontroller will
probably abort operation and turn off the laser based on a failure of the
focus as well as inability to return valid data after some period of time.

However, you may be able to cheat:

If the unit has a 'Test Mode', it may be possible to force the laser to
remain on despite a total lack of return signal - or even without the focus
and tracking actuators even being connected, for that matter. Many models
have a Test switch, jumper, or pair of solder pads on the mainboard (enable
before powering up). Then, there may be a key sequence to enable the laser,
move the sled, etc. See the document:
Notes on the
Troubleshooting and Repair of Compact Disc Players and CDROM Drives for
more information.

Where such a feature is not provided:

First, whatever is used to detect a disc must be defeated. Usually, this
is a reflection of the laser (most common).)but may be a separate sensor.

Then, the 'focus ok' signal must be provided even if you are not attempting
to focus the laser beam. It may be possible to tie this signal to the
appropriate logic level to do this.

Even if it is not possible to access these signals, depending on design,
you may be able to locate the logic signal to turn on the laser and enable it
there. However, some systems bury this inside a chip based on the controller
to activate it. Getting a schematic will probably be essential - but this
may be difficult (especially for a CDROM).

CAUTION: Take care around the lens since the laser will be on even when there
is no disc in place and its beam is essentially invisible. See the section:
Diode Laser Safety before attempting to
power a naked CD player or simlar device.

It may be easier to just remove the pickup entirely and drive it directly. Of
course you need to provide a proper laser diode power supply to avoid damaging
it. See the chapter: Diode Laser Power
Supplies for details. You will then have to provide the focus and/or
tracking servo front-end electronics (if you need to process their signals or
drive their actuators) but these should not be that complex.

Some people have used intact CD player, CDROM, and other optical disc/k drive
pickup assemblies to construct short range interferometers. While they have
had some success, the 'instruments' constructed in this manner have proven
to be noisy and finicky. I suspect this is due more to the construction of
the optical block which doesn't usually take great care in suppressing stray
and unwanted reflections (which may not matter that much for the original
optical pickup application but can be very significant for interferometry)
rather than a fundamental limitation with the coherence length or other
properties of the diode laser light source itself as is generally assumed.

In any case, some of the components from the optical block of that dead CD/DVD
player may be useful even if you will be substituting a nice HeNe laser for
the original laser diode in your experiments. Although CD optics are optimized
for the IR wavelength (generally 780 nm), parts like lenses, diffraction
grating (if present and should you need it), and the photodiode array, will
work fine for visible light. However, the mirrors and beamsplitter (if
present) may not be much better than pieces of clear glass! (DVDs lasers are
635 to 650 nm red, so the optics will be fine in any case.)

Unfortunately, everything in a modern pickup is quite small and may be a bit
a challenge to extract from the optical block should this be required since
they are usually glued in place.

Interferometers Using Two-Frequency Lasers

Interferometer-based techniques are used in all types of systems requiring
precision measurement of position, velocity, angle, straightness, and many
other parameters. These are part of a class of what are called metrology
applications. Examples include semiconductor wafer steppers in
photolithography systems,
hard drive and CD/DVD/Blu-ray disc mastering, optical diamound turning and
other high performance CNC machines, general machine tool calibration, and
many more. Measurements can be made over 10s of meters with resolution down
to nanometers using the wavelength of a known wavelength of laser light
as the meter (or yard) stick. Before discussing systems using
two-frequency lasers, we need to back up.

Note that all the techniques being discussed are for measuring displacement
(or position change), not absolute position. Absolute measurements are
possible using lasers but require additional techniques that are beyond
the scope of this discussion.

There are two classes of measurement interferometers called "homodyne" and
"heterodyne". The homodyne approach uses a single-frequency laser and
compares the phase of a reference and a measurement beam directly to compute
displacement (or position change). The laser is typically a stabilized
HeNe laser and the optics are in a configuration
like a Michelson interferometer. These systems are probably
best where the change in position (for example) is relatively small (a few
cm) and keeping cost to a minimum is important. One suitable application
would be in a hard drive servo writer. A typical configuration is shown
in Interferometer Using Single Frequency HeNe Laser.
The laser can be any of the frequency stabilized HeNe lasers described in
the chapter: Commercial Stabilized HeNe
Lasers, except that the linearly polarized beam must be oriented at
a 45 degree angle with respect to the Polarizing Beam-Splitter (PBS).
Half of it then gets polarized horizontally (into the plane of the
diagram) by the PBS and is returned from the retroreflector of the
"Fixed Arm" as the reference beam (REF) while the other half gets
polarized vertically passing through the PBS and is returned from
the retroreflector of the "Test Arm" as the measurement beam (MEAS).
They are recombined in the PBS as a single beam that has two
components whose relative phase depends on the relative position of the
two retroreflectors, and this changes as the Test Arm is moved.
Some fraction of the combined beam goes to an "Intensity" photodiode that
produces an output proportional to the beam power. This is needed to keep
track of the actual signal level. The remainder is split into two parts
which go to separate photodiodes (PDs). One PD has a polarizer in front
of it which results in a sinusoidally varying output based on the relative
phase of the REF and MEAS beams. Call this the "cos" signal. The other
PD has a Quarter-Wave Plate (QWP) and polarizer in front of it. The QwP shifts
the relative phase of the REF and MEAS beams by 90 degrees so that the
PD now produces a signal that is shifted by 90 degrees in phase. Call
this the "sin" signal. (An alternative implementation
is to use a "special optic" before the photodiodes to convert
the phase difference between the reference and measurement beams
to a rotation in polarization. Then, only polarizers would be
needed in front of the photodiodes and it would seem to be much more elegant!
I assume that every first-year physics student knows what the
"Special Optic" consists of in their sleep.)
The sin and cos signals being in quadrature (offset from each other by
90 degrees) are sufficient to determine displacement (consisting of
both distance change and direction) using digital hardware
only slightly more complex than a common up-down counter. This
is the same type of hardware used with optical encoders
based on parallel lines or gratings, but with the interferometer
approach, using the wavelength of light itself.

The benefit of the homodyne approach is simplicity and low cost (at
least in a relative sort of way as none of these systems is exactly
inexpensive!). And for some applications, it's more than adequate.
The basic measurement processing is
little more than what keeps track of the position of a computer mouse.
The laser can be built very inexpensively (despite what it probably
costs the end-user!) and the optics and optical receiver are quite
rudimentary.

However, there are several deficiencies that make these systems undesirable
(or at least much more difficult to implement)
for more sophisticated applications. Since they are comparing the phases
of the REF and MEAS beams directly, the result at any given time is a DC
level that depends not only on the relative phase, but also on the actual
output power of the laser and optical losses elsewhere in the system, drift
in the electronics, and even very slight changes in optical alignment.
But the electronics does tend to be simpler and unlike the two-frequency
systems, the only upper limit on velocity is one of optical detection and
processing speed, not the value of the "split" frequency of the laser.

The interferometers described in the previous sections and found in physics
labs (assuming such topics are even taught with hands-on experience!) all
use CW lasers and look at the fringe shifts as the relative path lengths of
the two arms is changed. While this works in principle and has been used
widely, modern commercial measurement systems based on interferometry
often use more sophisticated techniques to reduce susceptibility to signal
amplitude changes and noise and improve measurement accuracy, stability,
and convenience. These are called "heterodyne" systems in which the
laser beams are in essense carriers for a lower "split" frequency
in the MHz range provided by the two-frequency laser. The split frequency
is detected optically, but then can be manipulated using straightforward
electronics totally in the AC domain. If you're totally confused by now,
never fear. There is much more below. ;)

The microchips in virtually all modern electronics (including the
CPU and memory inside the PC or MAC you're reading this on)
were likely produced on photolithography systems incorporating
wafer steppers using two-frequency interferometers for multiple axes
of ultra-precise motion control.

Interferometer-based measurements systems typically use some type of low
power stabilized helium-neon laser to produce the "yardstick" beam of light.
By stabilizing the laser with reference to the neon gain curve, the
accuracy of the optical frqeuency/wavelength can easily be known
to better than +/-0.1 ppm. As noted above, a basic system may use such a
laser in a Michelson or similar interferometer, with a quadrature (sin/cos)
detector to count fringes representing changes in path length as described
above. Problems with such a system are that changes in light intensity will
result in measurement errors, alignment is very critical to obtain adequate
fringe contrast, and they are more susceptible to noise.

In two-frequency interferometers, a special stabilized HeNe laser is used that
produces a beam consisting of two very slightly different frequencies
(wavelengths) of light simultaneously. This may be achieved by various
techniques. HP/Agilent lasers employ a special tube which uses a magnet
to perform Zeeman splitting while Zygo uses an external acousto-optic
modulator. As above, both types of lasers are locked in such a way
that the optical frequency is very precisely known.

The following description applies to the HP/Agilent approach using Zeeman
splitting. With Zygo, the method of generating the two frequencies differ,
but their use in the interferometer is the same.

In the Zeeman split approach, the two-frequency laser consists of a HeNe laser
tube surrounded by permanent magnets which produce a constant axial magnetic
field. The laser tube is short enough that without a magnetic field,
only a single longitudinal mode will normally oscillate if it is located
near the center of the neon gain curve. (Those on either side will not
see enough gain.) The net result of the magnetic field is that instead
of a single longitudinal mode, two modes are produced that differ very
slightly in frequency and have right and left circular polarization. The
difference between the two frequencies is typically in the 1.5 to 4 MHz
range (though some go up to 6 MHz or more), which makes the resulting signals
extremely easy to process electronically. The actual difference frequency
is determined by the strength of the magnetic field, length of the
internal laser cavity, and other physical details, as well as the
exact place on the Zeeman-split neon gain curve where the laser has
been locked.

To stabilize the laser, there is a piezo element and/or heater to precisely
adjust cavity length. A feedback control system is used to adjust the
cavity length to maintain the position of the Zeeman-split frequencies
- and thus the wavelengths - constant. The feedback is generally based on
the simple approach of forcing the orthogonally polarized outputs to be equal,
which results in the highest beat frequency and most stable optical
frequency.

The wavelength of the laser is the measurement increment ("yardstick")
and will remain essentially unchanged for the
life of the instrument. For example, with the doppler broadened gain curve for
the HeNe laser being about 1.5 GHz FWHM (1 part in about 300,000 with
respect to the 474 THz optical frequency at 633 nm) and a 1 percent
accuracy within the gain curve, the absolute wavelength accuracy will
then be better than 1 part in 30 million! Not too shabby for what
is basically a very simple system. In practice it's even better. :)

Note that the exact value of the difference frequency does not need to be very
precisely controlled over the long term. Rather, it is the difference
between the reference difference frequency and the measurement difference
frequency that matters, and the latter only depends on
the motion of the target reflector - and the speed of light. Thus, the exact
beat frequency of each laser need not be precisely controlled or even precisely
measured and recorded or used anywhere in the calculations.

Since the output of the laser is a beam consisting of a pair of
circularly polarized components, a Quarter-Wave Plate (QWP) and Half-Wave
Pate (HWP) are used to separate these into two orthogonal linearly polarized
components called F1 and F2, and to orient them such that they
are parallel to the horizontal or vertical axes.

The beam consisting of F1 and F2 is split into two parts with a non-polarizing
beam-splitter: One part goes through a polarizer at 45 degrees (to recover a
signal with both F1 and F2 linearly polarized in the same direction)
to a photodiode which generates a local copy of the reference frequency
(REF, the difference between F1 and F2) for the measurement electronics;
the second is the measurement beam which exits the laser. The return
beam is called MEAS.

The purpose of the remainder of the interferometer is essentially to
measure the path length change between two points. In a typical
installation, the beam consisting of F1 and F2 is sent through a
polarizing beamsplitter. F1 goes to a cube-corner (retro-reflector) on
the tool whose position is being measured and F2 goes to a cube-corner
fixed with respect to the beamsplitter and laser. However,
differential measurements could be made as well using F2 in some other
manner. Various "widgets" are available for making measurements of
rotary position, monitoring multi-axis machine tools, etc.

The return from the object corner reflector is F1+ΔF1 which
is recombined with F2 and sent to an "optical receiver" module - a photodiode
behind a polarizer at 45 degrees and preamp which generates a new difference
frequency, F2-(F1+ΔF1). This signal, called "MEAS" is compared with
REF to produce an output which is then simply ΔF1. (A mixer is shown
in the diagram but in practice, it might be implemented with digital logic
like counters and subtractors.) A change in the
position of the object by 316 nm (1/2 the laser wavelength) results in
ΔF1 going through a whole cycle. By simply keeping track of the number
of complete cycles of ΔF1, this provides measurements of object
position down to a resolution of a few hundred nm with an accuracy of
+/-0.02 ppm! And the typical implementation will multiply the REF and
MEAS frequencies by 16 or 32 or more using a pair of phase-locked loops
to produce a corresponding improvement in resolution down to a few nanometers
or better!

The primary disadvantage of heterodyne systems is that the maximum velocity
is limited in the direction that would reduce MEAS since going through 0 Hz
would be confusing at best. So, one of the key specifications for these lasers
is the (minimum) split frequency. For example, the HP-5517B has a split
frequency range of 1.9 to 2.4 MHz with typical samples being 2.20 MHz.
But the minimum is the critical value and for 1.9 MHz, the maximum velocity
will be around 0.5 m/s using the simplest (linear) interferometer. Zygo
lasers have a 20 MHz split frequency so the velocity can be over 10 times
higher.

There are generally low power HeNe lasers with either specially designed
(and expensive) laser tubes or an external acouto-optic modulator (also
expensive) to produce the two (relatively) closely
spaced optical frequencies with orthogonal polarization. Depending on
technique, the difference frequency can be anywhere from a few hundred kHz
to 20 MHz or more. Since the beat frequency between the reference
and measurement signals decreases with one direction of motion and can't
go below 0 Hz (or at least becomes confusing as it passes through 0 Hz),
a higher difference frequency translates into
higher maximum speed of position change in the measurement system.
Therefore, depending on the specific application, a higher difference
frequency may be essential.

Transverse Zeeman laser: A transverse magnetic field of a few
hundred Gauss aligned with the natural polarization of a short HeNe laser
tube will result in a pair of modes that are orthogonally polarized with
a difference frequency of a few hundred kHz. The laser can be stabilized
by locking this difference frequency, or by mode intensities. Due to the
low difference frequency with this approach, it has had limited use in
metrology applications. See the sections starting with:
Transverse
Zeeman Stabilized HeNe Lasers.

Axial Zeeman laser: An axial magnetic field of a few hundred to
a few thousand Gauss applied to a short HeNe laser tube will result in a
pair of modes that are right and left circularly polarized, and differ in
frequency by up to about 4 MHz. A Quarter-Wave Plate (QWP) is used to
convert these to orthogonal linearly polarized modes and, a Half-Wave
Plate (HWP) is used to align them with the XY axes. With the difference
frequency in the MHz range, maximum speed of motion is adequate for
applications like machine tools and wafer steppers. Lasers using this
approach are manufactured by Agilent (formerly Hewlett Packard), Excell,
and others. They are the workhorse of the two-frequency interferometer
market. See the sections starting with:
Hewlett-Packard/Agilent Stabilized HeNe
Lasers.

AOM modulated laser: By passing the single frequency beam from
a HeNe laser through an Acousto-Optic Modulator (AOM), sidebands are
created separated by the AOM modulation frequency. The main beam and one of
the sideband beams become the two frequency output beam. This approach
doesn't require a special HeNe laser tube, but the added complexity still
adds to cost, and the higher difference frequency also requires more
sophisticated processing. See the sections starting with:
Zygo HeNe Lasers.

The following discusses the various types of optical components supplied
by Hewlett Packard (now Agilent) for measurement of position (or more
accurately, displacement or change of position) or velocity (rate of change
of position). There are also other
optical configurations for measurement of angle, flatness, straightness,
squareness, and more. But in essense, all of these convert a change in the
measurement parameter into a change in position. So, the basic principles
of operation are the same. Optics from other companies like Excel Precision
and Zygo are similar.

While the description below deals with "AC" or "heterodyne" systems using a
two-frequency laser, the same optical configurations are also applicable to
"DC" or "homodyne" systems using a single-frequency laser. Aside from the
type of laser, the optical receivers (and subsequent processing) will also
differ. Teletrac (now Axsys) was one manufacturer of these generally
lower performance (and lower cost) systems.

The most basic application (for a single axis measurement) will consist of
the following optical components:

Two-frequency laser: This is a low power CW laser (usually HeNe)
with horizontally and vertically polarized longitudinal modes that differ
in frequency by anywhere from a few hundred kHz to 40 MHz or more. The
laser is stabilized for the optical frequency, though the difference
frequency doesn't need to be particularly stable, or even known precisely.
Much more on these lasers can be found in the chapter:
Commercial Stabilized HeNe Lasers under
"Hewlett Packard/Agilent Stabilized HeNe Lasers" and "Zygo Stabilized
HeNe Lasers".

Interferometer: The relative Doppler shift of the two frequency
components after returning from either a fixed or moving reflector
results in a change in the difference frequency and this is what's
produced in the interferometer and then compared with the fixed
difference (or reference) frequency from the laser. Interferometers
are combinations of a polarizing beam-splitter, 0 to 2 quarter wave
plates, and 0 to 2 retroreflectors (cube-corners) fastened together
in a very stable optical assembly. The specific type of interferometer
used depends on the application, physical constraints, and cost.
More on this below.

Reflector: The moving part whose position is to be measured
will have either a retro-reflector (cube-corner) or plane mirror. to
return the beam to the interferometer. This will be determined by
the type of interferometer used.

Optical receiver: The return beam consisting of the two
optical frequencies, one or both of which may be Doppler shifted,
is passed through a polarizing filter oriented at 45 degrees to a
silicon photodiode where they mix producing the difference frequency,
which is amplified and converted to a differential digital signal.
HP calls this "MEAS" and it is compared with "REF" in the electronics
to produce the actual position information.

Since the optical frequency/wavelength is being used as the "measuring
stick" in these systems, it must be known to a high degree of precision
and anything that affects it must also be taken into account. In
particular, the temperature, pressure, and humidity of the air must
be factored into the measurement calculations. Or, if part or all
of the measurement setup is in a vacuum, this will affect it. These
corrections can be done at least partially automatically with sensors, or by
manually entering parameters into the measurement computer.

These systems generally allow a single laser to be used with installations
where the motion of multiple access needs to be measured. So in addition
to the actual measurement optics, there will be components to split and
redirect the original beam from the two-frequency laser to each axis.

Beam-splitter: To distribute the original laser beam to the
multiple axes, 50 or 33 percent non-polarizing beam-splitters are generally
used. The exact split ratio isn't important as long as adequate laser power
is available for each axis. HP specs up to 6 axes from a single laser,
though it's not clear there is any real practical limit as long as
the returning beam seen by the optical receiver for each axis has
enough power in it. These are physically plate beamsplitters mounted
in robust metal cubes.

Beam bender: Due to the physical arrangement of the machine tool
or whatever, it is almost always necessary to either redirect the beam.
to maintain orthogonality of the two frequency components, this must always
be done such that the resulting beam change in direction is a multiple of
90 degrees. Any reasonable number of these may be used to achieve the
desired configuration as long as each one deflects or rotates the beam
by a multiple of 90 degrees. "Beam Benders" (as HP calls them) are simply
45 degree high reflectance dielectric mirrors mounted in robust metal
cubes.

Interferometers:

The heart of all of these systems are the interferometers. The three most
common configurations are shown in Types of Hewlett
Packard/Agilent Interferometers. (This diagram applies directly to
two-frequency lasers like the 5517 where F1 (the lower optical frequency)
is oriented horizontally. Where F2 (the higher optical frequency) is
oriented horizontaally as with the 5501A/B, simply swap F1 and F2 in the
diagram.) There can be various permutations of
the individual components that are optically and functionally
equivalent. Combinations of multiple interferometers
mounted on a common platform are also available for compact multi-axis
applications. What HP calls the "interferometer"
consists of all the components in the center of each diagram - the Polarizing
Beam Splitter (PBS), 1 or 2 Retro Reflectors (RRs), and 0, 1, or 2
Quarter-Wave Plates (QWPs). There will also be a RR or Plane Mirror (PM)
on the "tool" whose position is to be measured, and an Optical Receiver (OR)
for the return beam. The Two-Frequency Laser (TWL) can be shared among all
the axes of the machine by distributing its beam using non-polarizing beam
splitters and 45 degree mirrors ("Beam Benders", but all beam orientations
must be a multiple of 90 degrees to the original TFL).

As noted, all of these interferometers contain a high quality Polarizing
Beam Splitter (PBS) cube as their central component. What gets added on
to the PBS depends on the specific type and may include Retro-Reflectors (RRs,
which are solid cube-corners) and/or Quarter-Wave Plates (QWPs). Please refer
to the diagram, above. For HP-5517 lasers, F1 is the lower frequency
component and is horizontally polarized, while F2 is the higher frequency
component and is vertically polarized. (HP-5501 lasers are the opposite,
but the only effect is a sign change in the measurement calculation.)

Linear Interferometer (10702A) with Retro-Reflector (10703A):
This is the simplest and least expensive of the three types and is
used where space allows and the axes of a multi-axis machine are
totally independent of each other. Either the Tool RR or the
interferometer itself can move.

The 10702A is a high precsision PBS cube in a robust mount. The 10703A
is a solid cube-corner in a robust mount.

F1 beam path: From the TFL through the PBS to the Tool RR and back through
the PBS to the OR.

F2 beam path: From the TFL reflecting off the PBS to the Reference RR
and back to the PBS reflecting to the OR.

Plane Mirror Interferometer (10706A): This one is unique in that
the tool can use a simple planar mirror rather than a cube-corner, and
produce a usable signal even if the measurement beam is not at a perfect
right angle to the mirror because the second pass through the
PBS and Measurement RR (cube-corner) exactly cancels the angular error.
Either of the other two interferometers would impose an unreasonably
strict requirement on angular tolerance (between the measurement beam
and a plane mirror reflector) to be useful. And, of course, the mirror can
move from side-to-side or up-and-down without losing the signal. A feature
(or byproduct) of its design is that since the beam goes between the
interferometer and bounces off the mirror twice, the basic measurement
resolution is doubled compared to the other types, from 0.166 um
(6.25 microinches) to 0.0833 um (3.125 microinches). (And of course
more sophisticated processing can multiply this basic resolution by
an order of magnitude or more!) Isn't it convenient how
Mother Nature made 633 nm very close to 25 microinches (actually 24.92 and
a fraction microinches). So the 6.25 microinch figure is very close to
1/4 wavelength of the 633 nm HeNe laser light! I always knew there was
something inherently elegant about the red HeNe laser! :)

The 10706A consists of a high precision PBS cube, a pair of solid
cube-corners for the Reference and Measurement paths, and a QWP
between the PBS and Tool path, all in robust mounts. It is
physically similar (perhaps identical except for labeling) to a 10702A,
a pair of 10703As, and a 10722A "Plane Mirror Converter" - the QWP.

F1 beam path: From TFL through the PBS through the QWP to the Tool PM
(first pass), back through the QWP reflecting off the PBS to the Measurement
RR, back to the PBS reflecting through the QWP to the Tool PM (second pass),
back through the QWP through the PBS to the OR.

F2 beam path: From TFL reflecting off PBS to Reference RR and back to
PBS reflecting to OR.

Single Beam Interferometer (10705A) with Retro-Reflector (10704A):
This is so-called because the outgoing and return beams are coincident. It
is the most compact and is used where space is at a premium. It is
functionally similar to the Linear Interferometer except that the OR
is at right angles to the input beam path and the interferometer cube
cannot be the moving part in a measurement.

The 10705A consists of a compact high precsision PBS cube with QWPs
in the Reference and Tool paths, all in a robust mount. The 10704A
is a compact solid cube-corner in a robust mount.

F1 beam path: From the TFL through the PBS through the QWP to the
Tool RR, back through the QWP to the PBS, reflecting to the OR.

F2 beam path: From the TFL reflecting off of the PBS through the QWP
to the Reference RR, back through the QWP through the PBS to the OR.

The 10705A can be converted into a plane mirror interferometer with
performance specifications similar to that of the 10706A by removing
the bottom QFP and installing a second 10704A RR. A non-polarizing
beam-splitter (e.g., 10700A or 10701A) is then required to sample the
return beam for the OR. (Due to the presence of the beam-splitter,
there will be some loss in available power when using this configuration.)
However, this also implies that part of the return beam goes directly back
into the laser, which can be destabilizing. I do not know what the
implications might be.

The Agilent prices are also interesting in that they are at least 5 times
what similar optical components would cost from a supplier like Melles
Griot or Newport. I do not know how much of this is actually due to the
required optical precision compared to less demanding applications.

Another clever configuration is the Differential Plane Mirror Interferometer
(DPMI). With minor modifications, the DPMI provides either displacement or
angular measurements that are extremely precise because the optical paths for
both the reference and measurement beams are virtually identical. In addition,
the reference and tool mirrors themselves can be located in close proximity.
Thus the effects of environmental changes will be the same and not affect
the measurement accuracy. A diagram of the Zygo implementation is shown in
Zygo Differential Plane Mirror Interferometer
(DPMI). The Polarization Shear Plate (PSP) splits the F1/F2 beam into
a pair of parallel beams separate by about 9 mm. This is equivalent to
a Polarizing Beam Splitter (PBS) and 45 degree mirror or prism. The Half-Wave
Plates (HWPs) rotate the polarization axis of one of these beams to be
the same as the other. The remaining components - Polarizing Beam Splitter
cube, Retro-Reflector (RR), and Quarter-Wave Plate (QWP) handle each of these
beams in the same manner as the measurement (MEAS) beam in the Plane Mirror
Interferometer (PMI) described above. Each of the beams pass through the PMI
and gets reflected by either the Reference Mirror (RM), or the Target or Tool
Mirror (TM). But due to the QWP, the polarization axis is rotated 90 degrees
on the way back, so it reflects off the diagonal of the PBS down to the RR
and back up reflecting once again off the PBS to the TM. Finally, due to a
second polarization rotation of 90 degrees, it passes through the PBS cube
back to the PSP where the beams are combined in reverse, and then out to the
Optical Receiver (OR) typically via a fiber-optic cable. In short, the
purpose of all this complexity is to split the F1/F2 components apart so
that their path lengths can change independently and then combine them back
into a single beam.

The DPMI is the key component of the Zygo
Compact Wavelength Compensator (CWC), shown in Photo
of Zygo Compact Wavelength Compensator (CWC). (The cover protecting the
delicate mirrors was removed for the photo.) This device provides a
separate measurement whose value is precisely proportional to the change in
wavelength from a reference point due to changes in temperature, pressure,
and humidity. The resulting value can then be used by the processing
electronics to greatly improve the accuracy of all measurements.
The HP/Agilent 10717A Wavelength Tracker, which is functionally equivalent
to the Zygo CWC, is based on their version of a DPMI which has an extra RR
to configure the input beam rather than a PSP. Excel Precision apparently
offered a true refractometer rather than a compensator or tracker. This
provided an absolute wavelength reference and would thus be far superior
if it was accurate.

It should be noted that the exact same optical configurations are used
with single frequency (homodyne) interferometers. With those, f1=f2,
usually produced by orienting the linearly polarized input beam at 45
degrees.

The most basic requirement is to convert the phase shift between REF and
MEAS to a count or position. Conceptually, this is nearly trivial, being
simply the different between the total cycles of the REF and MEAS signals.
And in fact, the original HP-5505A display unit did this by brute force
with a pair of counters and a subtractor. As will be shown below, this
turns out to be a clean, but rather hardware intensive solution. In general,
care needs to be taken in the design of the processing hardware and/or
software to avoid possible errors ultimately resulting from the
Uncertainty Principle.

Possible approaches

Dual counters with subtractor: As noted above, this was the
original method used by HP in the 5505A display, which went with the 5500C
laser. To handle a +/-1 meter range (without resolution extension)
requires approximately +/-1.6 million counts corresponding to 21 bits plus
sign, or 6-1/2 digits plus sign. This is a fair amount of hardware is
implemented with discrete CMOS or TTL, but should fit nicely in a modern
FPGA.

With 2's complement or 9's complement arithmetic, as long as the difference
remains less than half the maximum (i.e., the sign doesn't flip), the
result should be correct for a subtract. Since both counters can be
triggered directly from REF and MEAS (with suitable input filtering and
limiting, there are no arbitration or sampling issues with the counters.
However, readout of the subtracted result must be done with care to avoid
the chance of catching the result during a transition. One approach would
be to read twice in rapid succession and only accept the value if the results
match. In addition, to avoid single-count oscillation even when nothing is
moving, the read should be referenced to either REF or MEAS, but not done
with a totally independent clock, and/or successive values should be compared
to previous values and only updated when there are two successive counts
in the same direction.

Digitally sampled REF and MEAS drive up/down counter: This is the
simplest for a strictly hardware-based solution. However, just generating
Up and Down clock pulses from the REF and MEAS signals can have potential
problems where they occur very close together. Commercial up/down counter
chips are not designed to produce unambiguous results if the two clocks
occur so close together as to violate their setup and hold specifications.
Sampling and synchronization to a system clock is required. But whenever an
independent signal is sampled by a periodic clock
in something like a D flip-flip, there is also
no way to guarantee it will satisfy the setup and hold time requirements of
the device. There will be a (hopefully) small window where the flip-flop
can enter a metastable state where Q and ~Q are equal and not recover for
an arbitrary unbounded amount of time which may as long as the sampling
period. This is contrary to what many logic designers assume.
In fact, there has been considerable research published
in scholarly journals on this topic! It's theoretically impossible to
eliminate this potential problem entirely, though with careful design,
the probability can be made so low as to be of no concern. Which devices
are more susceptible to metastable behavior is not something found in the
datasheets. For example, when the common 74xx74 D flip-flip was tested,
it was found that not only was the behavior dependent on the type (e.g.,
LS, AS, F, HC, etc.), but on the particular manufacturer as well!
The issue is that should the device enter the metastable state, its output
may only slowly recover, and when used as an input to subsequent logic,
not meet their setup or hold requirements! An error that occurs once
in a billion samples may not seem of consequence, but we're talking
about measurements that may be made over hours with clocks running
at MHz rates.

And, as with the first approach, some hardware or software should be
included to eliminate the single-count oscillation issue.

I built a pulse converter that is based on this approach. It includes
single-ended inputs for REF and MEAS, the provision for up to 4 stages
of shift register to minimize the probability of metastability occurring,
and single-count oscillation elimination. It even has a pair of monostables
driving LEDs so I can watch when even single Up and Down pulses occur!
It's truly amazing how sensitive a system that measures in micrometers
is to any vibration!

Digital Signal Processor based: With a suitably fast programmable
system, input conditioning, sampling, counting and other arithmetic can
be performed in firmware providing both a hardware-efficient design as
well as much more flexibility. With modern technology, 50 or 100 MIPS
DSPs are inexpensive and should be able to handle the required tasks
and many more without working too hard. :)

Counting cycles of the phase difference is fine for education and demos, but
real systems almost always implement some type of resolution enhancement
scheme like frequency multiplication using a Phase-Locked Loop (PLL) or
ultra-high speed sampling. The spec sheets can then claim a resolution
that is not simply a fraction of the wavelength of the HeNe laser, but
down a few nanometers or even better. It is not known to what extent
these spectacular specs are realizable and repeatable in practice over
the life of the system taking into consideration the normal decline in
laser power, accumulated dust on the optics, changes in alignment, and
other factors which result in increased noise in the optical signal.

I was tired of searching for something inexpensive on eBay and anyhow,
wanted something interesting to do. I also miss the days of 0s and 1s
a bit (no pun...) and my drawer of ancient TTL chips was getting
rather lonely. :)

REF and MEAS input: Although these are differential signals and
a differential receiver would provide higher noise immunity, for my purposes
a simple Schmitt trigger using a single-ended input is more than adequate.

Anti-metastability shift register (optional): To reduce the chance
of a metastability event to longer than n times the age of the Universe,
multiple D-register stages may be added. This is currently not present
on the schematic but would go between the input conditioning and single
pulse edge synchronizers. One option would be a 93L28 duel 4 bit
shift register.

Single pulse edge synchronizers: These produce a single clock
pulse-width output pulse synchronized with the system clock for each
rising edge of the REF and MEAS inputs.

Single-count oscillation suppression: Output count pulses are not
generated if (1) up and down pulses would be strictly alternating and
(2) if both up and down pulses would be simultaneous.

Up and down pulse outputs: These are separate negative going
triggers for the up/down counter.

Monitor monostables: Every digital system has to have lights! :)
Each count produces a 1 ms pulse - green for up and red for down!

7 stage decade up/down counter with display (not built yet): There
are many options for this implementation. The basic approach using multiple
4 bit stages of counter/decoder/display is appealing in that a variety of
types of parts (e.g., CMOS, TTL) can be used and they are readily
available. Thanks to John Fields for Seven Digit
Up/Down Counter with Display. An approach with fewer parts could
use the Maxim ICM7217 4 digit
up/down counter with built-in display drivers. For 7 digits, two
ICM7217s and 7 7-segment displays.

While the laser is warming up and there is no REF signal, only the green
LED is lit. Once REF appears but if the MEAS beam is misaligned or
blocked, only the RED LED is lit. Once everything is stable and aligned,
neither is lit if there is no movement. I also breadboarded a version
using a PAL to generate the control signals. This eliminates most of
the discrete logic chips.

Before I added the display, I had to be content watching
the green and red LEDs. Any vibration - including moderately loud sounds -
result in flickering. I'm not sure what type of music they prefer, but
a sustained tone can result in quite impressive activity. :-)

The next version of a measurement display will probably be based on an
FPGA for the front-end logic with an on-board microprocessor to handle the
units/format conversion, user interface, and communications with a PC.
But I don't expect that to happen any time soon, if ever. :)

Of course, this isn't exactly rocket science. Besides HP/Agilent, Zygo, Excel,
and the other major players, these things have been stuffed into LSI ICs
for a long time. One example of an almost single chip solution is from
Laser Metric Systems, Inc..
They have a much more limited line of metrology systems but do have
a PC ISA or PCI bus card (PRM-004 series) whose brains is a single
Altera FLEX FPGA that can handle the performance requirements of just
about any two-frequency laserinterferometer with a claimed resolution down to
0.1 nm. Well, maybe. :) See the product info on their Web site.

And check out this (open access) paper: "FPGA-Based Smart Sensor for Online
Displacement Measurements Using a Heterodyne Interferometer", Sensors 2011,
11, 7710-7723. They digitize the analog REF and MEAS signals in
dual 14 bit 20 MHz flash A/Ds. So, in addition to the basic computation
taking the difference of (wrap-corrected) accumulators for REF and MEAS,
they use the analog waveforms to refine the measurement and estimate
the actual phase difference (partial wavelength) claiming a resolution
of 3.4 nm over a range of 3 m.

While it sounds really impressive to be basing precision measurement
on the wavelength of light, and HP/Agilent lists the nominal wavelength
of the 5501 and 5517 lasers to 9 significant figures, there are
many environmental and installation factors that impact what is actually
useful. The following is the briefest of summaries of accuracy issues.
A large portion of the operation manuals for these systems is devoted
to this topic. More than you could ever hope to know can be found in
the links in the section: Agilent Laser
and Optics User's Manual. The following is a brief summary.

Laser wavelength accuracy

A common question that comes up with respect to these systems
is: "Since these are based on two-frequency lasers,
which frequency is used as the wavelength specification?".
The quick answer is: It's not clear. :)

A phase change of 360 degrees of the difference frequency of the measurement
beam with respect to the reference beam represents a change in position of
of the moving part of the measurement system (e.g., the "tool") by 1/2
wavelength (linear or single beam interferometer) or
1/4 wavelength (plane mirror interferometer). Thus the component (F1
or F2) that goes to the tool is the actual "yardstick" wavelength. If
both F1 and F2 are used in a differential measurement, then each contributes
to the measurement based on its wavelength. So, strictly speaking, one
should use those wavelengths in the calculation. But as a practical matter,
it really doesn't matter as the difference in frequency between the two
components F1 and F2 is so small compared to the optical frequency, that
the error introduced by using one or the other is way below the accuracy
specification for even the military calibrated versions of these systems.

It's not clear (at least to me) where the value of 632.991372 nm for the
5501B and 5517A/B or 632.991354 nm for the 5517C/D comes from.
My assumption would be that it's the theoretical lasing line center of the
Zeeman-split neon gain curve. Various sources list other slightly different
values for HeNe lasers. Wikipedia
has a page on the Meter
Measuring Unit that gives a value of
λHeNe=632.99139822 nm. And an HP Journal article on
the 5528A gives yet another value of 632.991393 nm for the 5518A, but
that tube should be essentially identical to the tube in the 5517A!
The actual fill pressure, ratio of He:Ne and
their isotopic mix, temperature, and other factors will
affect the exact wavelength. But why the different values even for
essentially similar lasers from HP/Agilent including lasers that are still in
production? The corresponding difference in optical frequency between
632.991372 nm and 632.991354 nm is about 13.5 MHz, so it's way beyond
the error due to whether line center or either F1 or F2 is used.
Perhaps, it has something to do with the newer tubes being filled with
pure Ne20 or Ne22 and the older ones having
a mixture to guarantee compatibility in legacy applications. How's
that for a wild guess? :) The difference is still under 0.03 ppm,
so it should generally not be a huge issue in any case.

The normal HP/Agilent lasers have a long term stability specification
of +/-0.1 ppm. The laser stabilization depends on the condition of the
electronics and may result in a small variation in this wavelength.
Even one power cycle to the next does not result in a precise return
to the exact same conditions due to the particular temperature, and thus
cavity mode number, at which lock occurs. But any variation will
only be a few MHz at most, well below the +/-0.1 ppm specification.

The Agilent info indicates that they will certify a laser to
MIL STD-45662 for long term stability of +/-0.02 ppm. MIL STD-45662 requires
traceability to a (wavelength/frequency) reference, which I would assume to
be something like an iodine-stabilized laser. However, this doesn't sound
like the laser design is necessarily any different, just that the optical
frequency of the specific laser is measured precisely, and the exact value
to even more significant digits than the those given above, is included in
the calibration report. With that number known, +/-0.02 ppm corresponding
to about +/-9.4 MHz, really shouldn't be hard to maintain.

Velocity of light

Ambient temperature, air pressure, and humidity all have a very significant
effect on the measurement. Approximately a 1 part-per-million change in the
velocity of light and thus measurement wavelength will result from:

A change in air temperature of 1 degree Centigrade.

A change in air pressure of 2.5 mm of mercury.

A change in humidity of 30 percent.

Since 1 ppm is 10X of the basic measurement specification for accuracy,
it is clear that these factors must be taken into account. The HP/Agilent
measurement systems have sensor options to allow this to be done
automatically, or the corrections can be entered manually.

And, if the "tool" is in a vacuum, that's roughly a 3 part in 10,000 error
due to the difference in the index of refraction of a vacuum compared to air!

Material effects

Depending on the construction of the equipment on which the interferometer
optics are mounted, the change due to thermal expansion and other effects
can be very significant resulting in serious errors if not taken into
consideration.

So, how accurately can the wavelength or frequency of one of
these lasers be determined outside of a NIST calibration laboratory
or Agilent test facility? It all really comes down to
having a standard of reference. Commercial instruments called wavelength
meters available from companies like
TOPTICA Photonics AG can have very
high accuracy, some down to 2 MHz (0.0000027 nm) or better. But that is
only if a more accurate reference is used for calibration not to long
before making measurements of the unknown laser. 2 MHz isn't very much
when it comes to optical frequency! So, even if you could afford one
of these expensive instruments or could borrow one, it would still need
to be calibrated against some reference!

The most common reference of relevance for this testing
would be a mode stabilized HeNe laser like a
Spectra-Physics 117A. Its frequency will have a long term stability of
10 MHz or less, but whether the absolute accuracy can be nailed down to better
than 50 or 100 MHz - about 0.1 to 0.2 parts-per-million (ppm) - is
questionable due to factors like the exact neon isotope(s) in the
gas fill and the precise point on the neon gain curve where the
laser is locked. So, a higher precision reference would be needed
to calibrate that!

However, some of the HP/Agilent lasers themselves may have their frequency
known to extremely high accuracy. For the normal commercial versions of the
5517 lasers, the spec'd accuracy is *only* +/-0.1 ppm, which is equivalent to
+/-0.0000633 nm (+/-47.4 MHz in optical frequency) with a nominal wavelength
of 632.991354 nm (for the 5517C/D; 632.991372 nm for the 5501B and 5517A/B
and some others even though the tube is virtually identical in all of
these lasers).

But there are some 5517 lasers that come with a "pedigree" - a report
including the wavelength of the specific laser measured to very high
accuracy under controlled environmental conditions.

The report for one particular 5517D laser included the following:
Temperature and humidity test conditions of 22.8 °C and 36.2%,
respectively, a locked output power of 409 uW (spec is 180 uW
minimum), a split (REF) frequency of 2.75 MHz (spec'd range is 2.4 to
3.0 MHz), and the actual vacuum wavelength of 632.9913662 nm (nominal
is 632.9913540 nm).

That difference of 0.0000122 nm is actually a rather large error, equivalent
to about 9.4 Mhz in optical frequency or about 0.02 ppm.

I don't know if it is even remotely possible to obtain information
for a specific serial number laser from Agilent unless you're the original
buyer, and I rather doubt it. After all, your good fortune on eBay is
hardly something that Agilent is likely to care much about! :)
But, it may be possible to obtain the information from the original
buyer if they can be tracked down. I was able to have access to one
of these lasers that I was repairing, and they provided the above data
from the calibration report. With this data, I was hoping to be able to
measure the exact wavelength/frequence of one of my HP/Agilent lasers
and then use it as the reference when I return the other one. However,
it turned out that the wavelength report didn't apply to this particular
sample but a similar one. Oh well.

However, age and use of the laser will affect the optical frequency
by enough to matter. So unless the laser is relatively new, any data may
be unreliable. The drift may be predominantly due to changes in the
He and Ne partial pressure. These both alter the width of the Doppler
broadened Ne split gain curves, and shift the center optical frequency.
End-of-life lasers - the type often found on eBay - would be most prone
to such effects. But even relatively young lasers will see a drop in
pressure that may be significant. One tip-off to a potential discrepancy
could be the REF frequency. From my observations and the comment by an
engineer who tests these lasers, the REF frequency tends to increase
slightly with use. This is consistent with a broadening of the gain curves
which will increase the mode pulling effect. (But, at
least the change is in the beneficial direction - the one that increases
the maximum measurement velocity specification, assuming the processing
electronics can handle the higher REF frequency!)

However, assuming you found a new-in-box Agilent laser with complete
documentation of optical frquency:

Some relevant numbers: 633 nm is about 474 THz based on the speed of light
of 299,792,458 m/s. So, 1 nm is 0.749 THz or 749 GHz and
0.0000001 nm is 74.9 kHz. 100 kHz is 0.000000134 nm or 1 MHz is
0.00000134 nm. The difference between 632.9913540 nm and 632.9913662 nm
is then around: 9.137 MHz.

With one of these lasers in hand, it's really quite straightforward at
least in principle to determine the exact frequency offset of another similar
laser by beating (heterodyning) the two outputs in a high speed photodiode
and measuring the difference frequency. Since frequency doesn't depend on
environmental conditions, nothing that happens outside the laser will affect
it. The resulting optical frequency value can then be divided into the
speed of light in the relevant medium (e.g., air at STP or vacuum) to
compute the exact wavelength.

My Two-Frequency Interferometer Laser Tester (See
Diagram of Two-Frequency Interferometer Laser
Tester and Photo of Two-Frequency Interferometer
Laser Tester) was modified to permit the beam from a second HP/Agilent
laser to be combined and sent to an optical receiver or biased photodiode.
A diagram is shown in Diagram of Test Setup for
HP/Agilent Laser Optical Frequency Comparison.
A Polarizing Beam-Splitter (PBS) was added, at first only on a
somewhat adjustable mount (normally used for Beam Splitters or Beam
Benders in these systems) and
could easily be installed and removed without tools. However, this
proved to have nowhere near the estimated 1/10th of a mR
precision needed to align the beams to obtain a beat signal.
So, a Newport MM1 kinematic mount with the PBS attached to it
was installed in its place. The second laser
itself is on a fully adjustable platform (3 screws), so the beams
can be lined up precisely.

The distance from each laser to the PBS is relatively closely matched,
so if they have the same optics (e.g., 6 mm), the wavefront curvature
should be similar resulting in minimal alignment issues and decent signal
signal amplitude. How much mismatched optics (e.g., 6 mm with 9 mm) will
affect this is not known though. But in the far field, the more significant
issue would probably be the loss of available optical power from the size
mismatch since the wavefronts should be quite close to planar.

A Half-Wave Plate (HWP) which may be installed in front of either laser will
rotate the polarization if needed to pass it through the PBS, with the
PBS itself serving to eliminate the unwanted F1 or F2 component. Then,
F1 or F2 from Laser 1 can be beat with F1 or F2 from Laser 2.
However, not knowing the range of possible difference frequencies likely
to exist for any given pair of lasers, the highest bandwidth HP/Agilent 10780
may not be adequate to capture the difference frequency without a lot of luck,
especially if comparing lasers with different spec'd nominal optical
frequencies like the 5517A and 5517C (12 MHz difference). So my original
intent was to replace the HP optical receiver with a Zygo 7080 which is
used with their lasers having a REF frequency of 20 MHz. Assuming a
maximum of a +/-0.1 ppm frequency offset with respect to nominal for
each laser, the difference could be up to 95 MHz, though I highly doubt
even half of this is at all likely. Nonetheless, some higher speed
photodiode detector may be needed in general. And my poor old 10 MHz
oscilloscope which had been dedicated to monitoring of HP/Agilent
REF and MEAS frequency signals was augmented with another equally
old 50 MHz scope. :) (As it turned out, for the tests I actually
performed on two specific 5517B lasers, the original 10780A was
quite adequate.)

Since F1 (the lower optical frequency) and F2 (the higher optical frequency)
have known orientations, being able to select each one will make it possible
to unambiguously determine whether the difference frequency between the
two lasers represents Laser 1 or Laser 2 being the one that is higher
in frequency.

Another approach would be to monitor the difference frequency as the
cube-corner or plane mirror ("tool") in the interferometer is moved;
whether it decreases or increases for a given direction of motion will
also unambiguously determine which laser's frequency is the higher one.

For maximum accuracy, both lasers need to warm up from a cold start
(not restarted) for several hours in an environment with a fairly
constant temperature. But of course, the activity during warmup is
in itself quite exciting. :)

Note that the stray magnetic field from one laser will change the REF
frequency of the other laser if they are close together, especially
if they are parallel to each-other. The frequency offset that is
introduced is only order of 1 or 2 percent of the reference frequency
at most. It may in fact not affect the optical frequency very much, if
at all. And even if it does, the magnitude should be of negligible
consequence. But any disturbance is still worth minimizing. A bit
more on this below.

There may also be second-order effects of external magnetic fields that
are not aligned with the laser's magnetic field. This will not only
change the strength of the axial magnetic field, but will also introduce
a transverse component to the magnetic field with unknown consequences.

Although it turned out that I don't have a laser with a known optical
frequency and had to return the one that was probably close, I used a
pair of healthy 5517Bs to perform the experiment. But nothing
is perfect! :) A few of the issues:

It might be expected that the PBS itself would be sufficient to eliminate
the unwanted frequency component even with no polarizer on the output of
the lasers but no such luck. I don't know if this was due to a slight
angular misalignment, or less than perfect extinction ratios of the PBS,
but the net result is that the wanted frequency and
a small amount of the unwanted frequency exit the PBS cube. So, polarizers
were taped to the aperture wheel of each laser and aligned with the
F1 and F2. However, once the desired beat signal was obtained, these extra
polarizers could be removed without any noticeable effect since the
beat signal between lasers was much larger than the residual signal resulting
from the unwanted frequency components.

The required alignment precision needed to obtain a beat between the
two separate lasers is is quite high. In order to even get close the
first time, I had to bounce the combined beam exiting the PBS off a
plane mirror to a cube-corner a couple meters away and then align
the two beams where they returned near the photodetector.
Then, the photodetector was moved into the outgoing beam and some very
careful twiddling of the PBS mount and the second laser mount optimized
the signal. I had been totally unsuccessful at obtaining any beat
signal without the cube-corner technique. Part of this was due to
not knowing whether the difference frequency was even within the
bandwidth of my scope or what exactly to expect. However, subsequently,
simply aligning the two lasers so the beams were concentric at the
beam splitter and 8 inches beyond it followed by typical local search
and optics wiggling techniques used for laser cavity alignment would
usually get it back without too many 4 letter explitives - looking
for a sudden jump in the scope signal, rather than lasing flashes.
However, providing a longer baseline by reflecting the combined beam
to a white card about 3 feet away was helpful, especially where the
two lasers beam diameters differed. Then, very gentle pressing of
either the PBS would generally produce a momentary signal if one
wasn't already present, and adjusting the screws on the platform
on which the second laser was sitting would maximize the signal level.
This turned out to be more precise than adjusting the PBS mount.
For lasers with the same beam diameters (e.g., 6 mm) and decent
output power (200 uW or more), it takes only a couple minutes to
do the alignment with success on the first try. For mismatched
beam sizes or very low output power, it may take 2 or 3 attempts.

Before successfully obtaining a beat signal, I had attempted to use a
Scanning Fabry-Perot Interferometer (SFPI, see the section:
Scanning Fabry-Perot Interferometers),
to determine if the optical frequencies of the two lasers were close
enough together for the bandwidth of my photodetectors and oscilloscope,
or the HP optical receiver. I put the combined beam into a Spectra-Physics
470-3 SFPI head (550 to 650 nm, 2 GHz FSR) and was easily able to obtain
the display of the modes of the two lasers. But this really doesn't work
to provide reliable relative frequency information unless the lasers are
already perfectly aligned with each-other. Any deviation from
identical alignment with a mode-degenerate SFPI such as the SP-470 results
in a position shift of the mode display, which makes it pretty useless
for this purpose!

The signal is quite weak even with alignment optimized. The lasers have a
locked output power of around 650 uW (Laser 1) and 450 uW (Laser 2). After
the polarizers, this is down to 220 uW and 150 uW. The beam area of about
27 square mm is about double the Thorlabs DET-110 detector area of just
under 13 square mm (3.6 x 3.6 mm) so these optical powers get cut in
half - 110 uW and 75 uW. And there's the polarizer at 45 degrees before
the photodiode reducing them still further, perhaps cut in half again
so 55 uW and 37.5 uW. The sensitivity of the silicon photodiode is roughly
0.4 mA/mW so the output current will be about 22 uA and 15 uA. Into 50 ohms,
this results in a total voltage of 1.8 mV (DC). My actual beat or difference
signal (AC) is between 1 and 1.5 mV p-p or roughly 0.35 to 0.5 VRMS, which is
darn near the theoretical since the sum has a similar amplitude (but never
gets past the PD) and the DC terms are also present.

With lower output power from one or both lasers and/or mismatched beam
diameters (which reduces the overlap area where beating takes place),
it's of course more difficult. But I've been able
to obtain a high enough signal amplitude to sync the scope where the laser
being tested produced only 35 uW with a 6 mm beam diameter, using a reference
laser that produced 560 uW with a 3 mm beam diameter.

There is some residual frequency modulation in at least one, and probably
both lasers. Even after very carefully adjusting the polarizers to
eliminate as much of the unwanted frequency components as possible, the
signal is not anywhere close to a pure frequency. There is a very
noticeable frequency modulation (between 0.5 and 1 MHz p-p deviation)
with a strong component around 40 kHz. This is almost certainly
the optical frequency varying, not an artifact of residual
unwanted F1 or F2. The amount of F1 and F2 that make it
through the polarizers and PBS would be well under 1 percent
of the total optical power, and the
beat signal is more than an order of magnitude larger than this.
The most likely source is from the switchmode HeNe laser power
supplies. In fact, measuring the voltage across a
1K ohm resistor in the return of the HeNe laser tube on another
5517 laser shows a very significant ripple at, guess
where? Very close to 40 kHz! While ripple in the laser tube
current is the most likely candidate to cause the modulation,
magnetically coupled RFI from the power supply inverter transformer
to the tube above it through the air may also be possible, though
exactly how this could happen given the thick aluminum surrounding
the laser tube is not at all obvious, but we are talking about
relatively miniscule effects here! Many but not all of these
lasers have some sort of what appears to be a shield above the
power supply, reason unknown. It may just be a heatsink since
some lasers have only a partial shield and it's not even above
where the inverter transformer is located inside the potted power supply.

Well, it turned out that both lasers had partial shields or whatever.
Just placing a 5 turn coil with one end connected to a 10X scope
probe (other end not connected to anything!) above each laser's HeNe
laser power supply resulted in a very strong signal at the expected
40 kHz rate, but the maximum amplitude was NOT under the shield but
at the other end of the brick. However, neither signal seemed to
correlate conclusively with the frequency modulation, so there may
be a combined effect from both lasers, or something else is going on.

Next I substituted a different HeNe laser power supply (same model) in
one of the lasers. That may have made a slight improvement either because
the current ripple was lower or because it was hanging outside the laser.
Then I cobbled together a Spectra-Physics 248 set to 3.5 mA and run on a
Variac with a HV adapter to the HP laser tube. The SP-249 is a linear supply
so no chance of 40 kHz switching noise, only 120 Hz hum that sneaks through
the regulator! Indeed, the extent of the frequency modulation appeared to
drop by roughly 50 percent, probably down to a p-p deviation of 250 kHz.
Adjusting the Variac also had a small but definite effect even with the
regulated supply, though it probably was thermal because there was a
noticeable delay. The difference in optical frequency would increase
or decrease by several MHz, and then gradually recover to its previous
value after a minute or so as the feedback loop compensated.

At this point, I'm fairly confident that the HeNe laser power supplies
are the principle source of the frequency modulation and that such ugliness
is simply considered normal for these lasers. Even though it's the
wavelength corresponding to the optical frequency that is used for the
measurement, an uncertainly of +/-250 kHz represents an error of only
+/-0.0005 ppm, which is 4 times less than even the short term stability
specification for the 5517 laser. But really, Hewlett Packard and
Agilent, it wasn't worth the extra $10 to specify HeNe laser power
supplies with lower ripple??? :)

Using a Half-Wave Plate (HWP), it was possible to select F1 or F2 of
Laser 1 (L1) to beat with F2 of Laser 2 (L2). The difference in optical
frequency changed from 5.0 Mhz (no HWP, L2F2-L1F1) to 2.7 MHz (HWP
oriented at 45 degrees for a 90 degree rotation, L2F2-L1F2), proving
that Laser 1 has the lower absolute optical frequency. (Laser 1's
REF or split frequency is 2.3 MHz.) It sure feels good when the
physics cooperates! :) (For convenience in trying to keep track of
things, the difference frequencies here are referenced
to F1 of both lasers, rather than the average of F1 and F2, which
strictly speaking may be more accurate. In most cases, this results
in a shift of less than 1 MHz.)

The difference frequency between F1 of Laser 1 and F1 of Laser 2
(L2F1-L1F1) after coming Ready can be anywhere from approximately
-2 MHz to +6 MHz depending on whether one or both lasers was started from being
cold. (Of course, before locking, the difference frequency can be
up to 1.6 GHz or so - the FWHM width of the neon gain curve!)
The beat then drifts by 4 or 5 MHz over the next hour or so. Typically,
if both lasers are started at the same time, having been off for more
than an hour, the difference frequency (L2F1-L1F1) just after locking is around
-1 MHz, goes down to -2 MHz, and then climbs to a final difference
frequency of around +2.6 MHz over the course of an hour or so. Pretty
impressive for systems running at 474 THz. That 2.6 MHz is less than
1 part in 100,000,000!

The temperature of the laser - or more likely the temperature of the
control electronics - also affects the optical frequency by slightly
shifting the lock point on the split HeNe gain curve. The data above
was with the lasers undressed. Installing their
covers resulted in the difference frequency dropping to
under 1 MHz after an hour but then it went through 0 Hz and
seems to have finally settled at around -2.3 MHz after 5 hours.
Removing the cover over the PCB on Laser 1 caused the difference
frequency to climb back up to +2.6 MHz, so a change of almost 5 MHz.
I swapped in the Control PCB from another 5517 laser just for grins and
giggles, The optical frequency after 10 hours from a cold
start moved to about -1 MHz with at least as wide a variation during
warmup as with the original Control PCB. I don't presently have a 5517B with
a newer digital Control PCB so I can't compare its stability to that of
the common analog Control PCB.

Finally, I swapped the beam sampler assemblies including the LCD switch
on Laser 2 and this resulted in a significant change in the difference
frequency - almost 5 MHz. It's possible that an LCD panel that is
starting to delaminate or degrade in some other way could do this,
but I have no reason to suspect that one of these was bad.

Given how small the difference frequency between two randomly selected
5517Bs is, could it be that these 5517B lasers really are close to the
nominal spec'd value of 632.991372 nm or 473.612234 THz? Possibly,
since the least significant digit of 632.991372 or 0.000001 nm is
about 0.75 MHz. Where's a NIST calibration lab when you need one?

I also did some very basic experiments with magnetic fields applied
to one of the lasers. Placing another similar laser along-side Laser 1, the
difference in optical frequency could be dramatically changed as the distance
or orientation of the two lasers was altered. But, most - but not all -
of the frequency change was eliminated in a few seconds as the control loop
readjusted the locked position. The initial response didn't seem to
be instantaneous, so perhaps it was something related to a slight change
in laser tube current or something else affecting the temperature of
the tube. Given that both the strength and symmetry of the magnetic
field was being affected by repositioning, it's not at all clear what was
actually changing. But everything else should be intuitively obvious. :-)

Not content to permit one of these lasers to rest wherever it pleases,
I've built a simple network to introduce a small offset into the
error signal driving the laser tube heater power amp to fine tune
the optical frequency. The circuit consists of a pair of 1K ohm
resistors feeding 4.7 V zener diodes from +15 VDC (TP8) and -15 VDC
(TP10) to ground (TP1). (These testpoints
represent convenient locations to attach the circuit.)
The regulated +/-4.7 VDC are probably not really needed but won't hurt.
A 25 turn 25K ohm pot connects between +/-4.7 VDC with its wiper
feeding a "gain control resistor", whose other end is the offset output.
The "Power Amp" test jumper on the Control PCB was removed and replaced with
a connector having a 2K ohm resistor to partially isolate the driving op-amp
(test header Pins 1 and 2, assuming pin 1 is on the left), and the offest
is introduced to pin 4 (which is shorted to pin 2 on the PCB). With a
33K ohm gain control resistor, one turn of the pot changes the optical
frequency by about 1 MHz, providing a range of more than +/-10 MHz. A
positive offset reduces the optical frequency on the 5517 laser. (It
would probably be the opposite for the 5501B.)

There's no need to bother with this for the 5501A as it has a "Photodiode
Offset" pot (R4) on the Lock Reference PCB, which essentially performs the
same function. (It's the square pot in the corner.) Normally, the pot is
adjusted to maximize the REF/split frequency, which automagically centers
the lasing position on the split neon gain curve. But it has quite a wide
range - at least +/-50 MHz, possibly much more.

It would be a simple extension to lock two of these
lasers together at 0 Hz with a PLL using the REF frequency of Laser 1
(L1F2-L1F1) as the reference input to a phase/frequency detector (a
flip-flip and some simple logic), and the difference frequency between
the lasers (L1F2-L2F1) as the "VCO" input. The phase/frequency detector
output would be the offset error signal fed into the power amp. The
locked state would then have L1F1 equal to L2F1.

Even simpler and actually more flexible would be to use a monostable and
RC filter to convert the L1F2-L2F1 difference frequency to a voltage, and
compare that with a set-point value to generate the offset error signal.
This would allow the difference frequency to be adjusted over a wide range
with closed loop control, including the case where L1F1 equals L2F1 (or
with trivial modification, where L1F2 equals L2F2).

Both of these schemes are left as exercises for the student. :)

There is still another annoyance. It is a slow variation in the difference
frequency with a period of a few seconds. It looks like sort of a dance with
the control loops of one or both lasers hunting back and forth a few hundred
kHz to and much as +/-1 MHz. (The values for the difference frequencies
above were averages.) The period is between 2 and 3 seconds and appears
to remain relatively constant. When I first noticed this
hunting behavior, I didn't know if was a peculiarity of the locking
electronics or from some external influence like the DC power supplies,
vibrations, drafts, or aliens attempting to communicate with Earth. :)
There is a digital clock with a period of 2.56 seconds on the laser Control
PCB but I didn't think it should be doing anything once the laser locks.
And it wasn't something associated with the original Control PCBs in either
laser since swapping them with Control PCBs from other lasers
didn't noticeably change the behavior. Nor did swapping
one of the HeNe laser power supplies on the off chance that the
switching frequencies were interacting in some peculiar way.

I also substituted a 5501B for Laser 2, so call it Laser 3. (The final
resting place for the frequency difference was around 2.5 MHz for L3F1-L1F1.)
The variation in difference frequency was still present, but its deviation
seems to be much lower, perhaps +/-100 kHz. The other thing that changed
was the power supply since I had to use a different one for the 5501B. So it
was possible that power supply fluctuations, origin unknown, could be
the cause. However, then I swapped in the power supply originally used
for Laser 2 to power Laser 1 and that made no difference. Thus, not the
power supply. I then substituted Laser 2 for Laser 1 and the large
fluctuations returned. So, they must be either associated with the
tube itself or the LCD switch, since
everything else of relevance (Control PCB and HeNe laser power supply)
had already been swapped with no effect on the frequency fluctuations.
And note that the combination of Laser 1 and Laser 3 did have some
of this, just not nearly as much with Laser 2. But, even though
it isn't pretty, this could still very likely be considered normal
since even the much greater variations of Laser 2 are below the
allowable specifications for these lasers. However, I then swapped
the tube from Laser 2 into another case which meant the HeNe laser
power supply and Connector PCB were different. At first, it looked like
the difference frequency variation was way down, below +/-50 kHz, but
over time as both lasers reached equilibrium, it climbed back up to
around +/-300 kHz. Possibly somewhat lower than before but nothing
conclusive. The difference frequency had also shifted down by about
5 MHz (L3F1-L2F1 of 6 MHz). Swapping the control PCB made little
difference, but swapping in the original beam sampler assembly
restored the difference frequency to its previous value of 1.5 MHz
but also restored the large fluctuations! So, then yet another beam
sampler assembly was installed and now the difference freuquency is
around 0.5 MHz, but the fluctuations are also way down at +/-50 kHz
and seems to be staying that way. So, it may be that the beam sampler
has the most impact on both of these issues. Seriously strange.....

In frustration, I finally did some of the things I should have done
originally - changing the digital clock speed, looking at some control
PCB signals and - gasp! - actually reading the service manual in more
detail! :) It's easy to select clock speed on 5517B/C/D lasers - there
is a convenient jumper for "Normal" and "High". Switching to High
immediately caused the hunting to be much faster. Parallelling some key
resistors to change the oscillator speed also had a similar effect.
So, the hunting was related to the digital circuitry but
how? Checking some key test points immediately revealed that my assumptions
were totally wrong and that the cause is a direct result of
the HP/Agilent implementation of the feedback loop using the LCD
switch to alternately select each of the two polarized modes, rather
than incorporating a polarizing beam sampler with separate photodiodes.
Before realizing the cause, I had assumed that the LCD switch was
alternating polarization states at 50 Hz. It's not.
The two states are (1) when there is no drive to the LCD and both
sides are at the same potential (passive state, polarization rotated 90
degrees) and (2) when the LCD is driven by a 50 Hz squarewave of opposite
polarity on each side (active state, polarization unchanged). Apparently,
the response of the LCD is slow enough that the active state is essentially
DC - it doesn't see the 100 Hz ripple. (At least that's the theory.
Given that the behavior of these lasers in terms of exact locked optical
frequency and slow speed oscillation in optical frequency is affected by
the specific beam sampler with its LCD that is installed, I wonder if
there is enough variation in the residual response to be the cause.)
The LCD switches from the passive to the active state with a period of
2.56 seconds! Just before each state change,
the appropriate sample-and-hold is latched with the photodiode voltage
corresponding to that state. This happens every 1.28 seconds. So the
hunting is a direct result of this digital artifact in an otherwise
analog system. It should be possible to eliminate or greatly reduce
the effect. A few of the possibilities are to simply run the system at
a higher speed once it's locked, to improve the sample-and-hold circuits
(e.g., bigger caps!), or to replace the LCD and S&H circuits entirely
with a normal polarizing beam sampler and dual photodiodes.

At least one mystery remains and it has to do with the REF (or split)
frequency behavior after locking. Starting at that time there is a
fairly long period of an hour or more where the REF frequency oscillates
by 1 or 2 percent up and down (period of several minutes), with a smaller
oscillation of a few tenths of a percent period of 20 or 30
seconds). For example, on Laser 1, it starts between 2.279 MHz and 2.308 MHz
(change of about 1.25 percent) and the deviation of the faster oscillation is
0.002 MHz (change of about 0.09 percent). The deviation
of these oscillations gradually declines until it becomes small or
non-existent, or the periods become very long so the changes are
not easily seen. The mystery is why there is no obvious corresponding
oscillation of the difference frequency between two lasers! One might
expect that the origin of the REF frequency oscillations is the optical
frequency varying about its nominal value causing the REF frequency
to change based on the lasing position on the split gain curve. But
while the difference frequency drifts during warmup, there is no
obbious correspondance, by eye at least, with the REF frequency variations.

I then ran a pair of these lasers for many hours to get an idea of the
longer term stability. In all cases they are Lasers 2 and 3 from the
above link, a healthy 5517B and 5501B. The first set of data is for
both from a cold start:

The "Balanced Frequency Difference" is the mean optical frequency for Laser 2
[(L2F1+L2F2)/2] minus the mean optical frequency for Laser 3 [(L3F1+L3F2)/2].
* Denotes interpolated values, not measured. Read: Guessed, because I
missed recording the value for that time slot! :)

So, approximately 2 hours was required to get within 0.5 MHz of the final
value. However, these are the average of several 10 second measurements
and there is an uncertainty of several hundred kHz for each one. There
is high frequency jitter at the switching frequencies of the HeNe
laser power supplies (40 kHz tyical), as well as at the PWM frequency
of the 5501B controller (50 kHz typical), and slow hunting back and
forth from the control loops of both lasers (a few seconds or more).

Next, I turned off Laser 2 for 2 hours to let it return to room temperature
without touching Laser 3, and then started Laser 2 up again:

So, it would appear that most of the frequency drift with warmup is due
to Laser 2, and Laser 3 has very little drift after the first few minutes
and for several hours after that.
I don't know if this is a characteristic of 5501Bs in general, or whether
Laser 2 is simply particularly slow to reach equilibrium. But based on
previous testing, other 5517s were not much better. The 5501B control
scheme does differ in subtle ways from that of the 5517s, though not
in anything fundamental. The cause of the increase in optical frequency
difference after about 8 hours is not clear but it does seem to have
settled in at the new location. It's unlikely to be anything
related to the lasers themselves reaching equilibrium as that should have
happened well before this time. The ambient temperature may be changing
as I have little control over that in my, um, lab. :) The limit cycle
seems to have a total deviation of +/-150 kHz or about 0.0003 ppm.

Recall that these are averages over multiple 10 second intervals. My
next objective was to reduce the short term variations, initially
those due to the switching noise from the HeNe laser power supplies in
the lasers. So, I fabricated adapters so that the tubes in both lasers could
be powered from Spectra-Physics 248 exciters, which are AC line powered HeNe
laser power supplies with linear regulators. The residual ripple in the tube
current is below the scope noise floor and undetectable, compared to
3 percent p-p from the VMI power supplies HP used. Based on the previous
test where I substituted an SP-248 in one of the lasers, I assumed this
would clean it up. Well, not quite.

The result was slightly less messy, but there was still serious frequency
modulation with deviation in the MHz range, now seeming to be at a bit over
50 kHz. Since I was beating a 5517B with a 5501B, the last possibility for
switching noise of this type was from the 5501B, which uses Pulse Width
Modulation (PWM) to drive the heater rather than a linear power amplifier.
It runs at around 50 kHz nominal, but that's not very precise as it is simply
determined by an RC network.

So, I swapped in another 5517B (Laser 1) and Voila! The beat is now a really
nice sinusoid with only a hint of fuzz. This is what a heterodyne signal
should look like! No wonder HP did away with the PWM driven heater in the
5517 lasers! Exactly how the ~50 kHz noise makes its way into the
optical frequency is not at all obvious. It would seem to be too high a
frequency to be a mechanical effect but the heater coil is bifilar-wound so
there should only be a very small magnetic field from it. But perhaps,
very small is still enough!

As further insurance, I have modifed a pair of SP-248s to reduce
the already essentially undetectable power line ripple by at least another
order of magnitude. For details, see the section:
Reducing SP-248 Current Ripple
I also plan to add a protection circuit to the SP-248s that will shut the
supply down if either the current exceeds 4 mA due to a fault or the current
drops out. I don't want a power supply failure to fry a tube!

Photo of Test Setup for HP/Agilent Laser Optical
Frequency Comparison is really ugly but quite functional. The scope
on the right shows the actual beat frequency of about 3 MHz between F1
of one 5517B facing left-right and F2 of the 5517B pointing toward the
back of the photo, in this case with their covers off. The Variac is
powering the two SP-248s sitting under it to reduce the AC input voltage
until I add the addtional RC filter. (Although they work at normal line
voltage, the voltage across the regulator pass treansistors is pushing
their limits at the low current.) The Thorlabs DET-110 can just barely
be made out in place of the HP interferometer. In front of that also
sitting on the butcher block is a polarizer wheel that may be installed
in either beam. And the HWP to select F1 or F2, and bounce
mirror for alignment, are on the table in front of the butcher block.
The HP-5508A is displaying "PR Error" because it isn't
getting the signals it normally expects.

Most of the high frequency FM modulation is gone but the slow
variation persists with the frequency difference still varying by at least
300 kHz and sometimes much more over the course of a few seconds. This
is very puzzling. I have substituted a healthy 5517A for each of
the 5517Bs with no change and even put the AC power line to the entire
setup on a Sola constant voltage transformer, again with no change.
The control loop is not quite totally linear due to the 100 Hz sampling
of the LCD polarization selector, but this should be a high enough
frequency not to matter. But we do know that the specific LCD has
at least some effect on this behavior so some interaction there is
not out of the question. It sort of looks like a limit cycle in a
chaotic system, but part of this is probably because both lasers
are doing it at an independent rate. If not due to the control
loop impementation, what else can it be besides the aliens attempting
to contact us? :)

These lasers were apparently designed to be good enough for the metrology
applications, but are far from what would be considered to have low noise
or a super narrow line-width. The following are only some of the possible
causes resulting in changes to or frequency modulation of the optical
frequency by as much as 1 MHz or even more, over various time scales.
The following applies directly to HP/Agilent 5517 and 5501B lasers
except as noted, but much of it also applies to other HeNe Metrology lasers:

HeNe laser power supply current ripple (40 kHz): The HeNe laser
power supply bricks are apparently standard models with no extra attention
paid to the current ripple, which I've measured at about 3 to 4 percent
peak-to-peak (p-p), which is actually rather dreadful. A typical
off-the-shelf HeNe laser power supply brick from Power Technology or
Laser Drive will have a spec of less than 1 percent p-p. But out of
8 HP laser HeNe laser power supplies I tested, only one had a much
lower ripple of around 1.5 percent p-p. The one had a different
VMI model number - 217 instead 148, but the same HP part number.
Go figure. :) Apparently, it is an upgraded version
but I don't know if anything else was changed besides the amount of ripple.
VMI would not tell me anything other than that it had been upgraded
and suggested I ask Agilent. :) The large current ripple of the VMI 148
results in a deviation in optical frequency that may exceed 1 MHz at a
fundamental of around 40 kHz. (Harmonics will also be present.)
There's also a VMI 373, found on a 5517D-C29, manufactured in 2004,
characteristics not known.

A ripple reducer circuit can be added to the output of the existing HeNe
laser power supply brick. An in-line filter to prevent conducted RFI
back to the DC power supplies should also be installed. Or an external
linear HeNe laser power supply can be used in place of it. Either
approach will virtually eliminate this issue.

HeNe laser power supply leakage magnetic field (40 kHz: I don't
know if this actually contributes to the 40 kHz FM but moving the HeNe laser
power supply brick outside the laser housing did seem to have some effect.
The inverter transformer inside the potted brick produces an easily
detectable by simply placing a coil of a few turns in its vicinity.

Moving the brick further away from the laser tube would probably be
sufficient. It also may be possible to add appropriate shielding.

Pulse Width Modulation (PWM) from control loop (5501B only, 50 kHz):
The heater driver in the 5501B only uses a PWM scheme, presumably to
reduce power dissipation in the electronics. The PWM IC runs at around 50
kHz. While the exact mechanism isn't clear, this introduces significant FM
of the optical frequency. It would seem that 50 kHz is too high a frequency
for there to be a significant thermal response, so it may be magnetic despite
the bifilar winding of the heater inside the tube. The deviation may exceed
1 MHz at a fundamental modulation frequency around 50 kHz. (Harmonics
will also be present.)

Converting the PWM signal back to a linear drive signal should eliminate this
source of ripple.

LCD switching frequency (100 Hz): The horizontal and vertical
polarized modes are sampled alternately at a 100 Hz rate, fed to
sample-and-hold circuits and a difference amplifier. There could still
be some residual ripple/noise with a fundamental frequency of 100 Hz though
this has not been seen.

It might be possible to run this at a higher frequency or to improve
the sample-and-hold circuits, but I don't really know how much of an
issue LCD-induced ripple or noise really is.

DC power supply ripple/regulation/noise (120 Hz or SMPS chopper
frequency): This could get passed through both the HeNe laser power
supplies and control electronics. However, the DC power supplies are
generally quite well filtered and regulated.

Replacing the HP/Agilent switchmode power supplies with linear models
(e.g., from Power One) might be desirable.

Vibration (a few Hz to kHz): Vibration will affect the laser tube
structure and mirror spacing but short of a magnitude-8 quake, probably
isn't significant.

Placing equipment like the 5508A display with its vibration-inducing fan
off of the optical table would be desirable.

External magnetic fields (DC to 60 Hz): Magnetic fields have a
significant effect on the Zeeman split frequency and some effect on the
optical frequency.

Locating other Zeeman lasers or additional sources of strong magnetic fields
more than a meter away should be sufficient.

Temperature (slow): While the spacing rod of the HeNe laser tube
is temperature controlled to maintain the stabilized mode balance, the
overall tube is not temperature controlled. So, the internal gas pressure
in particular can vary. There is a significant change in optical frequency
versus gas pressure for HeNe lasers.

Monitoring the ambient temperature and the corresponding optical frequency
shift would enable it to be factored out of the calculations.

Pressure (slow): The laser tube in these lasers is of generally
conventional construction with relatively thin walls. Even though it is
mostly potted in a rubbery material and enclosed in aluminum, this would
not isolate it from atmospheric pressure changes. However, normal pressure
variations are not likely to have a significant effect.

Monitoring the ambient pressure and the corresponding optical frequency
shift would enable it to be factored out of the calculations.

Control loop hunting (0.3 to 1 Hz): This is a relatively slow
periodic frequency shift with a deviation of up to 0.5 MHz or more that
I've observed with nearly all 5517 and 5501B lasers to a greater or lessor
extent. (This is not present in 5501A lasers.) It appears to
be related to the LCD switch (since swapping this component can alter the
amplitude of the frequency deviation) and the digital clock (since changing
its frequency also changes the hunting frequency).

Since the specific LCD installed seems to affect the amplitude of the
frequency deviation, it may not be inherent in the design. But I do
not currently have a solution other than to replace the LCD its
sample-and-hold with a normal polarizing beam sampler! If anyone
has more info on this phenomenon or any of the other issues
that should be listed, please contact me via the
Sci.Electronics.Repair FAQ Email Links Page.

Not included here are normal variations in the optical frequency that are
a result of the lasing process itself.

Scanning Fabry-Perot Interferometers

While the interferometers described in the previous sections have
many applications in diverse areas, the Scanning Fabry-Perot
Interferometer (SFPI) is specifically designed to make measurements
of the longitudinal (axial) mode structure of CW lasers.
It rates it's own set of sections both due to its importance and
because it is possible to construct a practical SFPI at low cost
without the need for a granite slab or optical table for stability.

The longitudinal mode structure of a laser is one of those concepts
that is often explained but not so often demonstrated. There are
a number of indirect ways of showing that it exists including
monitoring the beat frequencies between modes and looking at
the fringe patterns in a Michelson or other conventional
interferometer. But one of the clever ways of actually being able
to display the modes as they would appear in a textbook is to
use an instrument called a "Scanning Fabry-Perot Interferometer"
(SFPI). The term "Laser Spectrum Analyzer" (LSA) may also be used for these
instruments, but an LSA should not be confused with an "Optical
Spectrum Analyzer" (OSA), which is generally - but not always -
based on a very different technology, the scanning monochromator.
Most SFPIs accept a free-space laser beam. However, some - mostly
designed for telecom applications - may use a fiber-optic connector
for input - a fiber collimator inside the instrument generates a
free-space beam for the actual SFPI.

While an SFPI is conceptually simple and actually quite straightforward
to construct (at least in principle), even a basic system can display
detail in the longitudinal mode structure of a laser that represents
about 1 part in 50,000,000 compared to the optical frequency of oscillation
or wavelength of the laser. For a red HeNe laser, the "resolvance"
of such an instrument would be on the order of 10 MHz (out of 474 THz) or
0.013 picometers (0.000013 nm, out of 633 nm)! It's all done with
mirrors! :-)

However, an SFPI can only show the spectrum of a laser's output over a
limited range of wavelengths (determined by the SFPI's mirrors) modulo
a much smaller value called the "Free Spectral Range" (FSR). The
FSR is determined by the mirror spacing and is typically not more than a few
hundred times the resolution. An SFPI cannot measure the absolute
optical frequency or wavelength of a laser. That requires an instrument
like an Optical Spectrum Analyzer (OSA) or optical wavemeter, or comparison
(heterodyning) with a known reference laser. So, an SFPI is like a microscope
that can display a very small region of a laser's spectrum at very high
resolution. And depending on the specific application, the SFPI display
may only make sense if there is already at least some idea of what to
expect. :)

An SFPI can be used to view the mode structure of lasers where the gain
bandwidth is less than its FSR such as a HeNe laser (~1.6 GHz) or
ion laser (~5 GHz). However, an SFPI can usually be used to determine if a
laser is Single Longitudinal Mode (SLM) regardless of its gain
bandwidth since the chances of multiple modes being both stable in height
and frequency, and falling on top of one-another (modulo the FSR) so that
the display shows a single peak is very small. And if more than one SFPI is
available with different FSRs, and the displayed frequency offset of a
laser with multiple longitudinal modes is the same on both, then that's
an indication - though no guarantee - that the distance between modes
is less than the smaller FSR.

A number of companies currently (as of 2010) offer SFPIs including
Coherent,
Thorlabs, and
Toptika.
For only a few thousand dollars, one of these instruments can be yours.
Alternatively, it's possible to build something with very respectable
performance for a couple bucks.
Companies like Spectra-Physics, and TecOptics no longer manufacturer SFPIs
but their instruments (as well as those that are still in current
production) may show up surplus at very
affordable prices. SFPIs may also be called "Laser Spectrum Analyzers".
But they should not be confused with even more expensive "Optical
Spectrum Analyzers" which are generally scanning monochromator-based
instruments that do not even come close to the resolving power of
a typical SFPI.

An SFPI uses the optical transmission characteristics of a
specially designed Fabry-Perot (F-P) resonator as a very
selective filter to scan across the optical spectrum of
the laser. Any F-P resonator will have a transmission
behavior that has peaks and valleys based on optical
frequency (or wavelength). The peaks will be located
where the distance between mirrors is an integer multiple
of one half the laser wavelength. As the reflectivity of the
mirrors approaches 100 percent, the peaks become increasingly
narrow and the valleys increasingly flat and close to zero
transmission. This characteristic looks like that of a "comb"
filter which is very selective.

An SFPI consists of a pair of mirrors with relatively high
reflectivity (90% to 99.9% or more is typical) mounted
in a rigid frame. In most SFPIs, the laser under test (LUT) is
aimed into one end and a photosensor is mounted beyond the other end.
The coarse spacing and alignment of the mirrors can be adjusted by
micrometer screws. The axial position of one of the mirrors can also
be varied very slightly (order of a few half-wavelengths of the LUT)
by a linear PieZo Transducer (PZT). (Other methods of moving the
mirror can and have been used but the PZT is most popular.) By driving the
PZT with a ramp waveform and watching the response of the photosensor
on an oscilloscope, the longitudinal modes of the LUT can be displayed
in real time. In essence, the comb response of the SFPI is used
as a tunable filter (by the PZT) to analyze the fine detail of the
optical spectrum of the LUT. As long as the FSR (c/2*L except under
certain conditions, described below) of the SFPI is larger than the
extent of the lasing mode structure of the LUT, the mode display
will be unambiguous. Where this condition isn't satisfied, the mode
display will wrap around and may be very confusing. For example,
the common helium-neon (HeNe) laser has a gain bandwidth of about 1.5 GHz
and longer HeNe laser tubes will generally operate with multiple
longitudinal modes covering much of this range. Thus the
FSR of an SFPI to be used with such a laser must
be greater than 1.5 GHz, corresponding to an SFPI cavity length of
less than about 100 mm (assuming c/2*L). For Nd:YAG, the gain bandwidth
is about 150 GHz, which results in a required SFPI cavity length of
less than 1 mm! However, in practice, lasers don't necessarily lase
over their entire gain bandwidth, especially if specific steps have
been taken to assure single or dual mode operation (also called single
or dual frequency operation). For those - which include many useful
lasers - the requirement can be relaxed such that the FSR of the SFPI
only needs to be larger than the width of the expected mode structure.
And for a single mode laser, this would be only the width of the lasing
line itself. Therefore, in these cases, a long cavity low FSR SFPI will
result in the highest resolution.

Commercial scanning Fabry-Perot interferometers usually cost thousands
of dollars - or more! But it's possible to construct an SFPI that
demonstrates the basic principles - and can be even quite useful -
for next to nothing, and one that rivals commercial instruments for
less than $100.

The resolution ("resolvence") of a Fabry-Perot interferometer is determined
by the wavelength, mirror reflectance, mirror spacing, and incidence angle
of the input beam. For the following, we assume normal incidence (which
will be satisfied in most practical situations).

Consider an SFPI with a mirror spacing (d) of 80 mm and reflectance (R) of
99 percent at a wavelength (λ) of 632.8 nm (red HeNe laser):

Another measure of the performance of an interferometer or laser cavity
is the "finesse". This dimensionless quantity is the ratio of the
FSR to the resolution. In essence, for the SFPI, finesse determines the
how much fine detail is possible within one FSR. The reflectance finesse
is equal to π*sqrt(R)/(1-R) where R is the reflectance of each mirror (which
are assumed to be equal). For R near 1 as would be the
case in a useful SFPI, this reduces to π/(1-R). (Unless you're a stickler
for precision, which case use the previous equation!)
While other factors
will affect the finesse, this equation will be reasonably accurate for
a properly designed spherical mirror cavity. So, with a reflectivity of
99 percent for both mirrors, the finesse will be roughly 300. If the
FSR is 1.875 GHz as in the example above, the resolution will be
approximately 6 MHz, which is in agreement with that calculation.

Transmission of Fabry-Perot Resonator versus Optical
Frequency is a composite plot that shows how finesse affects F-P behavior.
The Transmission of Fabry-Perot Resonator Slide
Show has a separate plot for each value of finesse, which is less
confusing when finesse is high. In an SFPI, the cavity spacing rather than
optical frequency is varied, but the SFPI display of a single frequency
(single longitudinal mode) laser for a given value of finesse will look
similar to these plots. Where the laser has more than one frequency
as is typical of common (unstabilized) HeNe lasers, the display will
essentially be a summation of shifted and scaled versions of these
plots (much more below). Low finesse F-P cavities (often even lower
than a value of 1!), usually called etalons, may be used to select specific
lasing lines due to their effect on intra-cavity gain. However, to be useful
for an SFPI, a high finesse is desired to be able to resolve lasing lines
that are close together. The finesse of a general purpose
SFPI will typically range from 100 to 500. Higher values are possible but
require better quality more expensive mirrors. As they say, it's all done
with mirrors. :)

Other factors will conspire to reduce the useful resolution of a practical
SFPI. At modestly high mirror reflectivity (e.g., R=99%), these include
alignment, input beam diameter, and input beam collimation. As R is pushed
closer to 100%, the quality of the mirrors, their cleanliness, and internal
losses become increasingly important. But for the example above, even if
the actual finesse is worse by an order of magnitude compared to the theory,
it will still be possible to easily resolve the individual modes of any
common HeNe laser and probably even the nearly 2 meter long Spectra-Physics
model 125 (177 cm resonator, mode spacing of 85 MHz). This is a factor of
better than 1 part in 10,000,000 comparing resolution to optical frequency!

However, note that while textbooks will tell you that the peaks should get
through with little attenuation, this is probably not going to be true with
practical high finesse SFPIs. (At least not those you're likely to see!)
The amplitude of the peaks will depend critically on the quality of the
mirrors and of course, on the alignment. For "laser quality" dielectric
mirrors, I've gotten as high as 5 to 10 percent peak transmission for a
high finesse SFPI using mirrors with a reflectivity of 99.8%. I'm sure
this can be improved upon but even so, for a 1 mW laser, there is still
more than enough optical power at the output of the SFPI to produce a
nice display on most scopes using a 1:1 probe without a preamp.

(From: A. E. Siegman (siegman@stanford.edu).)

In evaluating the effect of losses in Fabry-Perot mirrors you really
have to distinguish between internal losses (or loss-equivalent effects,
like scattering) that are physically located "inside" the mirrors (i.e.,
inside the effective reflection plane of each end mirror), and external
losses that are physically located "outside" the effective reflection
plane, but still within the physical layer of the mirror.

Losses that are outside the mirrors are effectively just additional
external transfer losses in the system - they have the same effect
as if they were separate from the FP, so that they don't affect the FP
itself but just weaken the light before or after the FP.

Losses inside the mirrors (aka "internal" losses) are more serious
because they are exposed to the higher-intensity resonant fields inside
the F-P and therefore can significantly affect the finesse and peak
transmission of the FP.

Just measuring the net reflectivity and net transmission of the mirror
itself won't clearly distinguish between these internal and external
losses. Also, how you'd describe a situation where the losses are
distributed through a moderately thick mirror layer is something I've
never thought through; doing this would require a slightly more
sophisticated wave calculation of forward and backwave wave propagation
inside the finite-thickness partially absorbing mirror layer itself.

A major disadvantage of the general spherical F-P cavity is that super precise
alignment and control of the input beam size and collimation, along with
an intracavity aperture, may be needed to suppress higher order transverse
modes in the SFPI resonator. Though not present in a
TEM00 laser, higher order modes are almost unavoidable in the SFPI cavity and
may in fact dominate the display and render it completely useless.
Even if the time consuming steps required to eliminate the higher order
modes are taken, there will always be
uncertainty as to what is actually being seen. The flat-flat cavity
doesn't have this problem but suffers from disadvantages of its own,
mainly in the need for a well collimated input and very precise mirror
alignment to achieve high finesse and as a result, reflection of the
input back directly back into the laser, which may destabilize many
types of lasers.

One way to eliminate the transverse mode problem is to use a cavity
configuration called a Mode Degenerate Interferometer (MDI) in which the
higher order transverse modes have the same frequency/wavelength as
the TEM00 (longitudinal) modes and thus simply fall on top of them
in the display. Even though each peak in the display representing a
longitudinal mode of the input laser may actually be
built up of contributions from multiple transverse modes excited in
the resonator of the interferometer, the characteristics
of the individual longitudinal mode components in each of these
transverse mode are the same so the accuracy of the resulting display
isn't affected. (This should not be confused with the very different
situation of a laser having multiple transverse modes in
its output where the frequencies, phases, amplitudes, and
polarizations of the corresponding longitudinal modes in each
transverse mode may differ.)

Two practical arrangements that satisfy this condition are the (1)
spherical cavity (d=2*r) and (2) confocal cavity (d=r). The confocal
cavity has the larger finesse and is thus usually employed in SFPIs
since the finesse is a measure of Q-factor with respect to the FSR
or mode spacing, and thus higher finesse results in better resolution.
A planar cavity (r of infinity) doesn't support higher order modes at all
but and its theoretical finesse is double that of the confocal cavity,
but is generally a less desirable configuration due to alignment
and other issues (see below).

Note that the term "confocal" actually refers to any cavity where the
focal points of the two mirrors are coincident. However, only the case
where d=r is stable and thus useful for the MDI SFPI.

The frequencies of the transverse modes of a symmetric cavity
Fabry-Perot resonator are given by the following equation:

Thus the first term is simply the optical frequency of the laser while
the second term consists three parts: the longitudinal mode spacing or FSR,
the integer mode numbers, and a correction factor (<1) that depends
on the mirror RoC and spacing.

The interferometer will be mode degenerate when there are TEM00 modes that
have the same frequency as some of the transverse modes. The
requirement for this to be satisfied is for the inverse cosine term
in the equation above to be equal to π divided by an integer, l. Then
there will be "l" types of modes with one type - where (1+m+n) is equal to
1, modulo(l) - having the same frequencies as some TEM00 modes.
When (1+m+n) is not equal to 1, modulo(l), that mode will fall
in between the TEM00 modes in locations depending on (m+n)/l, modulo(l):

Confocal cavity: d/r=1 or cos-1(1-d/r)=π/2. Thus,
modes where m+n is even will be mode degenerate. Non-degenerate modes
will fall at the mid-point between integer modes which in effect, cuts
the FSR of the cavity in half to c/(4*d) unless only the modes where
m+n is even or m+n is odd are excited, but not both. This will only
happen if the input beam is very well aligned to the optical axis.

Intuitively, the FSR is cut in half (except if very well aligned) because
a beam entering off-axis undergoes 4 reflections rather than 2
reflections, doubling the cavity length. Similarly, the finesse is
cut in half because a round trip has twice the loss as a result. See

The most common configuration is shown in
Confocal Cavity Scanning Fabry-Perot Interferometer.
The Input Mirror and Output Mirror are identical in both Radius of
Curvature (RoC) and generally also in reflectance (R).
The laser beam enters from the left with reflections from the coated
surface of the Input Mirror, from a similar location through the input mirror
(4th pass), and at the bottom location through the Input Mirror (2nd pass).
The useful output is generally both beams that get through the Output
Mirror and are focused onto the photodiode by an (optional) lens
at the output. (The lens is unnecessary if the photodiode is large enough
to intercept both transmitted beams.
The input beam is shown way off axis to clarify the paths
traversed inside the cavity. Normally, it would be much closer to the
optical axis. However, the input beam can come in at a slight angle
as well. And most confocal SFPIs will add a focusing lens at the
input (not shown) selected so the focal point is approximately at
the center of the cavity taking into consideration the diverging
effect of the input mirror. This will improve the resolution.

Spherical cavity: d/r=(-1) or cos-1(1-d/r)=pi. Thus,
all modes will be mode degenerate.

Planar cavity: d/r=0 or cos-1(1-d/r)=0. Thus the value
of q alone determines the mode. (This is probably not really
considered an MDI configuration since there are no stable higher
order modes at all!)

While the confocal and spherical MDI configurations are the best known
and most widely used, it's possible to make use of cavities having values
of l other than 1 or 2 and they may be useful for certain applications.
See: Variable
Free Spectral Range Spherical Mirror Fabry-Perot Interferometer. Though
that's for the advanced course, here are a couple of examples:

Long radius spherical cavity: d/r=(1/2) or cos-1(1-d/r)=π/3.
Non-degenerate modes fall at the 1/3 and 2/3 points between integer modes.

Long radius spherical cavity: d/r=(1/sqrt(2)) or
cos-1(1-d/r)=π/4. Non-degenerate modes fall at the 1/4,
1/2, and 3/4 points between integer modes.

There have also been approaches using what might be termed "semi-mode
degenerate" cavities, where the first few undesirable transverse modes
merge with the TEM00 mode, but others (which have much lower amplitudes)
may not. Some examples can be found in U.S. Patent #5,418,641:
Fabry-Perot Optical Resonant Cavity Systems. granted to Newport
Corporation in 1995. But I am not aware of any SFPIs employing anything
other than planar or confocal cavities, even from Newport (who now
has none at all).

Further investigation of these special cases is left as an exercise
for the reader. :)

For the confocal cavity, half of the transverse modes are not
mode degenerate when an on-axis input beam is used as
there are two types of modes depending on whether the quantity
(1+m+n) is even or odd:

(1+m+n) is even. These are not mode degenerate but fall halfway between
the other modes. The overall mode number is an integer.

(1+m+n) is odd. These are mode degenerate and have corresponding TEM00
modes. The overall mode number is an integer/2.

This seems a bit strange that the TEM00 modes (m+n=0) have non-integer mode
numbers but the equation has been confirmed from at least two different
sources.

As noted, with two sets of peaks, the FSR is effectively cut
in half to c/(4*d). Rearranging the equation above with the new FSR of c/(4*d)
out in front, one sees that the various transverse modes (those that
differ in m+n) result in a frequency difference of c/(4*d). However,
integer differences in q corresponding to the longitudinal modes, still
have an FSR of c/(2*d). Where a paraxial beam (one parallel to the
optical axis) enters the confocal cavity off-center, the beam path repeats
itself after two traversals of the cavity (in a zigzag pattern) and the
FSR is easily seen to be c/(4*d) rather than c/(2*d). However, if the
beam is very well aligned and centered, the FSR will be c/(2*d) since only
some symmetric modes will be excited. However, the finesse is still the
same (with respect to the 4 pass round trip cavity).

Note that when adjusting the mirror distance to be confocal, there will be
many positions where the SFPI may appear to work but which aren't quite
confocal. This is especially true of short confocal cavities - the type
most commonly found in commercial instruments.
Depending on the specific distance, non-degenerate higher order
modes will result in ghost peaks and/or a variation in the amplitude of
the lasing modes depending on their position on the voltage ramp drive
signal. The amplitude will also be lower overall. However, when the
correct distance is approached, all of these ghosts will collapse into
the desired high amplitude display. Don't be fooled! Thus it's best to
know or determine the exact RoC for the mirrors before installing them in
the SFPI so the initial distance can be set reasonably precisely.

Planar mirrors may also be used since a true flat-flat cavity does not
support stable higher order modes, degenerate or otherwise, but it is
the most difficult to align. And, although the theoretical finesse is
double that of the confocal cavity, the realizable finesse is usually
lower and is also much more dependant on the alignment than
with the confocal or with other non-planar configurations. Also, with optimal
alignment, the incident beam is reflected directly back into the laser
which may result in instability for many types of lasers. So, it's often
always necessary to use an optical isolator of some type (Faraday or
polarizing beam-splitter with Quarter-Wave Plate (QWP), or at least an optical
filter to reduce the intensity of the back-reflected beam (by the square of
the transmission coefficient). However, where
the distance between the mirrors of the SFPI is adjustable as in some
general purpose instruments like the TecOptics FPI-25 or the (likely) custom
Burleigh Triple-Pass Scanning Fabry-Perot Interferometer described below,
there is no choice. Both of these enable the distance between the mirrors
to be varied from almost touching (for an FSR of 100 GHz or more) to
15 cm or more (for an FSR of of 1 GHz or less).
(Intracavity etalons also usually use planar mirrors
but the finesse of these does not generally need to be very high
so the alignment is not nearly as critical.)

Some useful things to keep in mind:

The FSR is only a function of the distance between the mirrors and
the speed of light: FSR=c/2L. So, it's constant regardless of the
wavelength/frequency of the laser being tested. (Henceforth called the
"Laser Under Test" or LUT.) Think of changing from n to n+1 or n-1
half constant wavelengths between the mirrors.

The distance required to move the mirror 1 FSR is 1/2 of the wavelength
of the LUT so it increases as the wavelength increases. For example
at 633 nm, 1 FSR is achieved with a movement of 316.5 nm. But at a
wavelength of 3,391 nm, it is 1,695.5 nm - over 5 times greater.
In both cases, the same span of optical frequency has been covered.
To cover 1 FSR at the lower frequency, the movement is greater - with perfect
reciprocity, and vice-versa. So, the PZT and/or scan driver must be able to
accommodate this where multiple wavelength lasers are to be tested.
(As does the wavelength range of the photodiode.)

Where a laser produces multiple wavelengths that are widely spaced
(greater than the FSR of the SFPI), this will apply to each one of them
independently. Needless to say, interpreting an SFPI display of such a
laser can be, shall we say, challenging. :)

Any electrically adjustable fabry-perot resonator can be used in
a variety of ways including as a tunable filter or etalon.
However, our interest here is mostly with respect to their application as
a measurement instrument: The SFPI used as a laser
spectrum analyzer. Nowadays, there are even F-P devices
based on MEMS (Micro Electro-Mechanical Systems) technology. A
complete device may be extremely small, but as such, they will be
restricted to larger FSRs. Their flexibility will also be limited
since all adjustments *must* be done electrically.
Excluding these presently more exotic techniques,
there are three common types of SFPIs that one is likely to find in
an optics lab. These have been around and substantially unchanged for
several decades:

Description and comments: General purpose instruments
designed for interchangeable planar mirrors to select wavelength
range with widely variable mirror spacing to change FSR.

These are not really what's best for most applications unless multiple
diverse types and wavelengths of lasers are being used. They are large
and heavy, difficult to set up and adjust, and their performance for any
given set of parameters is often inferior to the others. However, where
a large FSR is required due to the laser's characteristics, there may be
no choice. As a practical matter, the plane-plane configuration is the
only one that can be set up for an FSR above about 30 GHz.

Advantages: Totally general purpose instrument.

Disadvantages: Not optimal for most often used applications,
extensive setup and alignment, large and heavy. Mirrors are exposed
to contamination and errant fingers or tools.

Examples: TecOptics FPI-25 and Tropel 350. These instruments
are beautifully constructed and have all the bells and whistles one
could ever want. A variety of mirror sets are (or were) available and
since they are planar, custom mirrors are relatively easy (if expensive)
to manufacture. The FSR may be varied continuously from a few GHz
to 1,000 GHz or more. But setting up one of these instruments for
a measurement can take hours.

Description and comments: SFPI heads with interchangeable
confocal mirror sets to select wavelength range and a small number
of FSRs (typically 2 to 4).

These are the most widely used type. The interchangeable mirror sets enable
the relatively expensive head (spacing tube with PZT and photodiode assembly)
to be converted to a different application by swapping mirrors. However,
as a practical matter, this is somewhat tedious and time consuming so
it's likely that most go through their entire life with the same mirrors.

Advantages: Easy to set up and align, mirrors sets may be swapped
to change wavelength range and/or FSR, small and light weight.

Disadvantages: Limited wavelength range and FSR choices. Confocal
cavity FSR limited to about 30 GHz max; less for most manufacturers.
May require adjustment after awhile.

Examples: Burleigh SA Plus, Coherent SA, Spectra-Physics 470,
and TecOptics SA. (Only the Coherent SA is a current product, and it
is likely being phased out.) Except for the SP-470, they have a
number of wavelength ranges covering most of the VIS range and
some near-IR. The Spectra-Physics 470 has wavelength ranges of
450-550 nm or 530-650 nm, and FSRs of 2 GHz or 8 GHz. So, switching from
an argon ion laser to a HeNe would require a different mirror set but
not an entirely different SFPI: 450-550 nm, 8 GHz FSR for argon ion;
530-650 nm, 2 GHz for HeNe.

Description and comments: SFPI heads with non-interchangeable
confocal mirrors with a fixed wavelength range and FSR.

These have the benefit that if manufactured carefully, they will have
the best long term performance due to more precise mirror centering and
spacing, likely clamped, locked, glued, and sealed. :)

Advantages: Easy to set up and align, never need adjustment,
small and light weight.

Disadvantages: Fixed wavelength range and FSR choices. Confocal
cavity FSR limited to about 30 GHz max; less for most manufacturers.
If not set up perfectly at the factory, no way to adjust. The warranty
has definitely run out by now. :)

Example: Spectra-Physics 450 with wavelength ranges of
450-550 nm or 530-650 nm, and FSRs of 2 GHz or 10 GHz. So, switching from
an argon ion laser to a HeNe would require a different SFPI head:
450-550 nm, 10 GHz FSR for argon ion; 530-650 nm, 2 GHz for HeNe.

In all cases, a separate ramp generator is used to drive the PZT and an
oscilloscope is used for the display. Commercial units will typically
include the ramp generator since the PZTs in most SFPIs require several
hundred volts of drive to cover a few FSRs. The 'scope must be
provided by the user. The ramp generators can be of various levels
of sophistication, some including fine tuning of alignment for the
general purposes SFPIs with triple-PZTs. A few SFPIs like those from
Thorlabs have a requirement of
only 5 V/FSR. Home-built SFPIs can be of any of the three types,
but (2) is easiest to construct if appropriate mirrors are available.
When using a beeper element as the PZT, a function generator (with at
most a simple op-amp circuit to boost the voltage) can serve as the
ramp generator since their drive requirements are modest.

Subsequent sections cover home-built as well as common commercial SFPIs.

I have used commercial general purpose Scanning Fabry-Perot Interferometers
(SFPIs). For example, the TecOptics FPI-25 is an example of a very solidly
constructed precision instruments with adjustments for just about everything.
However, being so general, in some sense it is not optimal for anything!
(See the section: The TecOptics FPI-25
Scanning Fabry-Perot Interferometer.)
There are somewhat less flexible but easier to use SFPIs from companies
like Coherent,
Thorlabs, and
Toptica
Photonics. These provide the following:

A confocal cavity so alignment is not nearly as critical as with the
general planar-planar configuration. Some include irises on the input
and output to make this even easier using the built-in photodetector.

A choice of mirror sets covering the visible and near-IR
wavelengths as well as several FSRs to enable high resolution (e.g., 1.5 GHz)
or large coverage (e.g., 30 GHz). Some allow for the mirror sets to be
swapped by the user.

A resonator structure with close to zero Coefficient of thermal
Expansion (CoE) which is mostly insensitive to temperature variations.
This it typically a material like Invar, or a combination of two materials
in a configuration so that their CoEs cancel.

They also have a price tag to match - those from Thorlabs
start at around only $2,400 not including the driver box (around
$800), others are even more expensive. (Several are covered in subsequent
sections, below.) You don't want to ask about the prices of the very
flexible SFPIs. :)

My challenge was to prove that I could construct an SFPI that would
at least demonstrate the basic principles and possibly even be useful.
The results are described in this and the following sections.
All of mine cost me absolutely nothing (except time) but
that wouldn't sound as credible as $1.00 or $2.00 or $3.00. :)
Yet many aspects of their performance are comparable to the
multi-$k commercial SFPIs. And in some cases, far superior.

The heart of the SFPI is its two mirrors. For longer visible
wavelengths (i.e., 600 to 700 nm), the mirrors can be the OCs salvaged from a
pair of dead red (632.8 nm) HeNe laser tubes. For other wavelength ranges,
mirrors from green (532 nm) DPSS lasers, green or blue ion lasers, HeCd, and
other lasers may be useful. While some of these mirrors may have a relatively
broad band reflectance, this cannot be counted on. More often than not,
the reflectance falls off dramatically beyond 10 or 20 nm from the spec'd
wavelength. And, obtaining proper single mode performance
of the SFPI without great pain may require that mirrors with specific
reflectances and RoCs not normally found in common lasers be used.
Of course (gasp!), suitable mirrors can be also be purchased.
For common wavelengths, they may be available from companies like CASIX at
very reasonable prices. But in general, obtaining the optimum mirror might
require ordering a set of custom mirrors. It's not the ground and polished
mirror glass itself that will cost a lot of money. They can often be
standard concave lenses with suitable curvature available from places
like Edmund Industrial Optics or Melles Griot. It's the custom coating,
which can easily exceed $1,000, and it doesn't matter that much whether
the lot is 2 mirrors or 200 mirrors as what counts is the coating machine
time. So, find 99 friends who want to build the same SFPI and the
per-mirror cost could still be quite low. :)

For a short RoC confocal cavity SFPI (more below), the only readily available
mirrors I know of are either the misfits I'm using in my $3 SFPI for HeNe
lasers (also more below) or mirrors from flowing dye lasers. Unfortunately,
the latter tend to have ground, but not polished, outer surfaces. However,
since the outer surfaces aren't critical, simply using some index-matching
fluid, optical cement, or even common oil or water, between the ground surface
and a piece of glass like a microscope slide or cover slip is know to work
well enough. It's the coated mirror surface that's important.

As far as attempting to coat your own mirrors - in two words: Forget it. :)
Unless you have access to a dielectric mirror coating machine and know how
to use it (and are permitted to use it!), there is no way to produce coatings
that will do anything more than provide a hint of what's possible. Metal
(aluminum, silver, gold) coated mirrors do not work well since their maximum
reflection coefficient is around 94 to 97 percent and they have high
absorption losses. Thus finesse will be poor and the photodetector
signal will be very small. And except for gold, the coatings degrade
(tarnish, oxidize) in air without a protective layer, with silver being
the worst. For good quality dielectric mirrors, absorption losses only
become a major concern for very high reflectivities (perhaps above 99.9%)
and modern coatings do not degrade significantly under normal conditions
as long as they are not subject to physical abuse or improper cleaning
techniques.

When specifying the mirror RoC (r) for a particular application, it
usually makes sense to base it on the maximum frequency range over
which there will be action, not simply on the gain bandwidth of the
laser(s) being observed. Not only will this result in the best
resolution, but doing otherwise may simply not be practical.
For common gas lasers like the HeNe and argon ion which
have longitudinal modes filling most of their gain bandwidth,
(1.5 GHz and 5 GHz, respectively) there's no choice if the
display is to be unambiguous. But where the modes have already
been limited by an etalon or some other means, only the range
of the modes that are present need to fit into the SFPI's FSR.
For example:

To display the modes of a normal (unstablized) HeHe laser, an FSR of
1.75 to 2 GHz is generally optimal, providing the best resolution (for
a given finesse) with an unambiguous (not aliased or wrapped) display.
These mirrors would have an RoC of around 43 mm for a confocal FSR of
1.75 GHz. A reflectivity of 99 to 99.7% would be acceptable.

To display the modes of a dual frequency Zeeman
split HeNe laser, an FSR much smaller than the 1.5 GHz gain
bandwidth is desirable. Rather than the 1.75 or 2 GHz
that would be considered optimal for the normal HeNe laser,
an instrument with a confocal cavity constructed using 60 cm,
99.75% mirrors would result in an FSR of approximately 125 MHz
(c/4*L for the confocal spacing) and a resolution of under
1 MHz. Of course, building such a beast could have considerable
problems of its own. :) For a more convenient setup, mirrors with
a somewhat smaller Roc could be used, with an acceptable sacrifice
in finesse. The mirrors salvaged from a pair of the same model
of HeNe laser barcode scanner tubes (typically 20 to 30 cm RoC,
99%@633nm) would be satisfactory. See the section:
Sam's High Resolution Scanning
Fabry-Perot Interferometer.

To confirm single frequency operation of an Nd:YAG or Nd:YVO4
laser like the Coherent Compass 315M or Uniphase uGreen, as long
as the FSR of the SFPI and laser are not related by an integer,
any additional modes popping up will be immediately obvious.
So, an SFPI FSR of more than 150 GHz resulting in a microscopic
cavity is not needed.

I have a variety of inexpensive mirrors suitable for 633 nm SFPIs available
on Sam's Classified Page.
These include short cavity mirrors that result in a truly spectacular
finesse at 633 nm. (FSR of 1.7 GHz, finesse may exceed 500!)

The other major components of the SFPI include the PieZo Transducer (PZT) to
move one of the mirrors a few microns, and a photodiode to monitor the
output beam.

High quality PZTs can be purchased at exorbitant cost. But the beeper from
a digital watch or similar device will work nearly as well and has the
advantage that it runs on much lower voltage than most other types. You never
did like that alarm anyhow. :) But there is no need to discombobulate your
watch as these piezo elements can be purchased from electronics distributors
or surplus places for about $1.00. :) While they aren't quite as linear or
have as good a frequency response as the high priced units, these deficiencies
don't really matter much for an SFPI. And since they will move several microns
on only 50 V, a high voltage amplifier isn't needed as with many commercial
SFPIs. The 20 or 30 V p-p output of a typical function generator or simple
op-amp circuit is quite adequate.

.
The photodiode can be almost anything since it needs neither a large area or
high frequency response. The salvaged photodiode from a barcode
scanner with a 10K ohm resistor load and 10:1 or 1:1 scope probe is often
adequate. Where more sensitivity is needed as with very high-R mirrors
or low power lasers, a trans-impedance amplifier with high gain can
be added since frequency response isn't that critical. Almost any common
op-amp will suffice, expecially if multiple stages having modest
gain (e.g., 5 to 10) are used.

Everything else is hardware. The structure and mirror mounts are easily
home-built. However, one area where it may be hard to compete with
commercial SFPIs is in minimizing the effects of temperature. They typically
construct the main support as a cylinder or set of rods made from Invar,
a low coefficient of thermal expansion alloy. Some designs further
compensate for residual effects by balancing them against those
of the PZT resulting a near zero net change in FSR with respect
to temperature and/or may include a heater in a closed-loop temperature
stablization system. Invar stock is available or can be salvaged from
various dead lasers. Some people build SFPIs by mounting the back mirror
and PZT in an Invar tube, positioning the front mirror using a 5-axis
lab stage, and then gluing it in place permanently when the optimal
mirror spacing and alignment has been determined. But glue tends to be
too permanent for my taste. :) Constructing the SFPI using Invar rods
is nearly as good. But simply enclosing a non-Invar based SFPI in an
insulating box will go a long way in reducing temperature effects.

And if the objective is to achieve a high finesse and maintain it, then
enclosing the entire SFPI to prevent mirror contamination is essential,
if not during operation at least for storage. While testing
my high resolution SFPI, the finesse had been steadily declining over a
few days which turned out to be dust collecting on the mirrors even though
their coated surfaces were vertical. Any cleaning of high quality
mirrors is to be avoided. Even when using the proper techniques for cleaning
of laser mirrors, some permanent degradation of the dielectric coatings is
virtually unavoidable. With many cleanings - or only one if the proper
techniques are NOT followed - the damage will be enough to result in a
noticeable decline in performance. (For information on laser mirror
cleaning techniques, see the section: Cleaning
of Laser Optics.)

A triangle (or sawtooth) wave source (it can be a simple circuit constructed
for this purpose or a general purpose function generator) and oscilloscope
(preferably dual trace and/or with an X-Y display mode) will be required to
view the scan but needn't be dedicated to the SFPI, so they don't count
toward the cost!

The next few sections include general descriptions and photos of several
home-built SFPIs. Schematics for both a photodiode preamp and simple
function generator are provided later in this chapter.

(From: A. E. Siegman (siegman@stanford.edu).)

When thinking about producing small and not too fast mechanical motions
or pressures, consider also magnetic methods.

After University Labs in Berkeley introduced the first really low-cost
lasers in the early 1970s (priced at circa $300 each rather than the
prevailing several thousand dollars and up), it also produced a really
neat and equally inexpensive little scanning F-P interferometer with
plastic end plates and the scanning mirror driven by what was in essence
a miniature loudspeaker coil.

One of the advantages of the magnetic versus piezoelectric approach is low
voltage, higher current drive circuitry, perfectly adapted to IC or
semiconductor electronics. Another advantage is wider range of motion.

This is the first of several SFPIs I've constructed, differing
mostly in the mirrors and their spacing. It uses curved mirrors but
is not mode-degenerate, having been built before I knew about such things. :)

The basic design is shown in Home-Built Scanning
Fabry-Perot Interferometer 1. My prototype uses the OC mirrors from
a couple of dead Aerotech 1 mW HeNe laser tubes. The PZT is the beeper
from some sort of musical greeting card with a 4 mm hole drilled in the
center. The photodiode is from a barcode scanner.
The frame and mounts are a bit different than those shown in the
diagram, above. They were made from the platter clamping plates from some
ancient 5-1/4" harddrives, hex spacers, and miscellaneous scrap metal.
The circular plates are nice because they have predrilled holes with
6-fold symmetry thus simplifying construction. See
Photo of Sam's $1.00 Scanning Fabry-Perot
Interferometer. (For the mirrors, /V denotes Concave, /P denotes
Planar.) Here is a summary:

The front mirror is removable so other reflectances or RoCs can be tried. The
rear mirror is glued to the PZT. The hole was made by placing the PZT
on a hard surface (e.g., an aluminum plate) and drilling through it slowly
with modest pressure using a normal metal bit in a drill press. The piezo
material is more of a compressed powder than a true ceramic so it's possible
to grind it away (using the metal drill) with minimal chipping. Thin flexible
wires were already attached but if they aren't, solder the top lead near the
edge to leave room for the mirror and to minimize any change in elasticity of
the top surface. Once soldered, Secure the wires mechanically with a drop of
flexible adhesive like 5-minute Epoxy. Also note that the metallization tends
to disappear with even modest heat or stress so solder quickly. Conductive
paint or silver Epoxy can be used to touch up bare spots if needed but use
as thin a layer as possible as it may increase stiffness and reduce response
sensitivity in that area. For this reason, DO NOT coat the entire surface
with adhesive of any type!

To perform initial alignment, I used a yellow-orange HeNe laser thinking it
would be easier since the mirrors are less reflective away from the
632.8 nm design wavelength. The scatter off of the mirror surfaces was used
as the initial means of setting alignment, by minimizing the size of the line
or blob formed by the multiple reflections. With a pair of concave mirrors,
not only do they have to be aligned with respect to the input beam, they also
have to be aligned with respect to each other. In other words, their optical
axes must coincide which requires walking them until the scatter pattern is
minimized. When misaligned, it will be a line or circle and no amount of
adjustment of only one mirror may improve it. Once the initial alignment
was done, the PZT could be driven and the output of the photodiode used to
fine tune it. In retrospect, using the funny color HeNe laser wasn't
necessary as enough red light gets through to be easily seen for alignment
purposes. And the display of the modes of that multi-wavelength and
multi-transverse mode laser was definitely strange.

The preliminary results using a Melles Griot 05-LHR-911 HeNe laser were
also confusing. This is a 2 mW laser using a tube with about 165 mm between
mirrors, corresponding to a mode spacing of 883 MHz. The scope trace in
Sam's SFPI Display of Melles Griot 05-LHR-911 HeNe
Laser - Initial Attempt shows a jumbled mess due to many transverse modes
being excited in the SFPI. The trace on the left should cover a span of
approximately three FSRs of the SFPI - about 19.5 GHz. Three clumps that look
about the same are clearly visible but the complexity isn't real. The trace
on the right is an expanded region of the one on the left. A hint of the
modes of the laser can be seen but only a hint. The 05-LHR-911 should have
2 or 3 longitudinal modes at most but the short cavity of the SFPI using
long radius mirrors is resonating with multiple transverse modes.

There is also some hysteresis in the PZT response. It's barely visible
on the display as the pattern differs slightly on the positive and negative
slopes of the triangle driving function. Using X-Y mode on the scope would
show up the hysteresis more clearly. Reducing the sweep speed slightly
virtually eliminates the hysteresis. (A 20 trace/second display has
minimal hysteresis and is still quite usable. Of course, this wouldn't
be an issue with a digital scope

The overall linearity of the PZT is around 5 to 10 percent over a range of
+/-20 V, corresponding to 5 or 6 FSRs of the SFPI. I've actually tested
several PZTs (another one was from a digital clock for which the alarm was more
of a nuisance than useful!). The response of one is compressed more toward
the upper end of the voltage range; the other is slightly compressed at both
ends. Within a single FSR, the linearity is probably better than 2 percent
and a range of a single FSR provides all the information usually needed.
For a system of this type where qualitative information is most important,
perfect linearity, especially over multiple FSRs, really isn't a major issue
in any case as long as it is known and doesn't change over time. A third
PZT was quite linear but had a range of only around 1 FSR of the SFPI -
probably due to the excessively thick layer of silver Epoxy I used to cover
some bald spots on the piezo disk.

To confirm that transverse modes were the cause of the complex display and to
partially remedy the situation, I aligned the SFPI more carefully by adjusting
the front mirror so that the 05-LHR-911 beam bounced directly back to the
source with dancing interference patterns, then aligned the rear mirror
for maximum amplitude of the displayed signal, and added an aperture about
0.3 mm in diameter (a pin hole in a piece of aluminum foil) inside the SFPI
cavity. The aperture was mounted on a micropositioner but could be installed
permanently so that doesn't blow my budget. :) The results are shown
in Sam's SFPI Display of Melles Griot 05-LHR-911 HeNe
Laser. The sequence of the six traces
show the modes of the 05-LHR-911 cycling over time as they
move under the HeNe gain curve. The horizontal scale is the same as
in the jumbled mess trace, above, but the transverse modes have been
almost entirely eliminated. The distance between similar peaks (2.2 boxes
on the screen) is the FSR of the SFPI - about 6.5 GHz. The distance
between longitudinal modes (0.3 boxes) is the 883 MHz FSR of the 05-LHR-911.
The math even works. :) So, this represents success of sorts
but alignment of everything is super critical and any vibrations - even
the audio from a radio - create havoc with the display. There is also
a quasi-periodic fluctuation in amplitude of all the displayed modes with
no corresponding power fluctuations in the laser. I suspect this to be
due to residual mode competition in the SFPI as the frequency of the modes
changes relative to the SFPI cavity, possibly a side effect of the aperture.

Finally, I tried a Spectra-Physics model 117A HeNe laser head, which when
used with its mating controller is a frequency or intensity stabilized
(single longitudinal mode) laser. I'm running it on an SP-248 so it's
not stabilized but the modes are a bit interesting. The mode spacing is
around 600 MHz which is consistent with a 2 to 3 mW HeNe laser. However,
as the modes cycle, there isn't a smooth progression through the gain
curve. It almost seems as though having exactly 2 modes is enhanced somehow
and that it's very unlikely to see 1 or 3 modes. When 1 or 3 modes would be
expected to pop up, they might appear very briefly, or be skipped
entirely in favor of the 2 modes one of which is on the opposite
side of the gain curve. The polarizations of the modes also appear to
be of the "flipper" variety, changing suddenly rather than staying with
a particular mode. I don't know if this behavior is by design. However,
since orthogonally polarized modes are sensed by a pair of photodiodes
in the laser head and used for stabilization, strong mode pairs could be
beneficial.

After determining experimentally that an aperture helped but didn't
totally eliminate the transverse mode problem, a Post Doc in our lab wrote
a simple Matlab program to calculate Hermite Gaussian transverse mode
profiles given the mirror RoCs and the distance between mirrors. Plugging
in the long radius SFPI cavity configuration revealed that the TEM00 and
TEM10/01/11 modes have a high degree of overlap regardless of axial position.
So, any aperture that suppresses them very effectively would also result in
unacceptable attenuation of the TEM00 mode. So, on to plan B. :)
I hope to have a compiled version of this program available in the near
future as it appears to be quite useful for visualizing cavity modes in
general.

Here is a summary of the configurations I've tried so far on the $1.00
SFPI:

Long radius spherical cavity - medium finesse (99%/V-99%/V, RoC of
20-30 cm): Both mirrors were the OCs salvaged from 1 mW Aerotech
HeNe laser tubes. (This is the original configuration, more info above.)
Alignment is fairly easy and forgiving. The resolution is better than
50 MHz based on the widths of the peaks compared to the FSR of the
05-LHR-911 HeNe laser. A 50 MHz resolution is about 1 part in
10,000,000 - not bad for something that costs $1. :)

Long radius hemispherical cavity - medium finesse (97%/P-99%/V):
The front mirror was replaced with a 97 percent reflectance planar OC which
I believe originated from a Spectra-Physics dye laser. The behavior of this
configuration was very similar to the original one using the pair
of HeNe OCs except that the resolution was slightly lower due to the
lower reflectance (97 versus 99 percent) of the front mirror. An aperture
cleaned up the resulting display in a very similar way as well.

Flat-flat cavity: (97%/P-99.7%/P): The rear mirror was replaced
with a near-HR (probably 99.7 percent or so) mirror from some sort of HeNe
laser based laser printer. Remember what I said above about high
quality adjustments? Mine are definitely marginal in this respect and
alignment was super critical. I never could obtain a resolution anywhere
near that of either of the previous configurations. This was likely due in
part to my inability to perfectly align everything and the likely
lower quality (flatness) of the rear mirror. However, the transverse
mode problem was indeed eliminated. :) (I later revisited this
configuration. See the section: Sam's
Plane Mirror Scanning Fabry-Perot Interferometer.

Long radius hemispherical - very high finesse 1 (99%/V-99.8%/P):
The rear mirror was replaced with the HR mirror from a HeNe laser tube. It's
a planar mirror with a reflectance probably better than 99.8 percent. The
front mirror was the original 99% concave mirror used previously. The very
low transmission of the HR mirror made alignment difficult and cleanliness
of both mirrors was critical. But even with the almost perfect rear mirror,
the signal was still reasonably strong (between 1/4 and 1/2 of the others) -
testimony to the high intra (SFPI) cavity power at resonance! This
configuration indeed had a significantly higher resolution than any of
the others - better than 25 MHz, possibly better than 10 MHz.

Long radius hemispherical - very high finesse 2 (99.8%/P-99%/V):
This is the opposite of the one above. While a signal could be obtained,
alignment was extremely difficult, mainly because so little light gets through
the front mirror (less than 4 uW for my 2 mW HeNe laser) and thus the main
alignment aid until the detected signal is large enough to be seen - visible
scatter from the mirror surfaces - is almost non-existent.

Of these, the first is probably the best choice unless super high resolution
is needed. All except the flat-flat required an aperture inside the SFPI
cavity to suppress non-TEM00 (transverse) modes.

Well, it wasn't actually $2.00. :) I found some small radius mirrors
originally intended for a research project that is now defunct.
These should work well in a confocal configuraion in the green region
of the spectrum free of those annoying transverse modes!

The mirrors were actually Melles Griot plano concave lenses custom
coated (along with a batch of microchip laser crystals) for 1,540 nm.
Now, it's perhaps a not so well known fact that a dielectric mirror
coated for a wavelength of X nm will also perform reasonably
well at a wavelength around X/3 nm (think of a stack of 3/4λ layers
instead of 1/4λ layers). The actual reflectance function will depend
on the design of the original mirror including the number of layers
and uniformity of the layer thickness. The reflectance at the new
wavelength will almost certainly be lower and the losses may be slightly
higher. But with luck, these mirrors will be useful in a wavelength range
centered around 513 nm (1,540/3).

I had two types available: Those that were supposed to be 98 percent
as OC mirrors and those that were supposed to be HR mirrors, both at 1,540 nm.
Here are how they performed at the two green wavelengths of interest:

For 532 nm, neither is really ideal. The "OC" is a bit low - I would have
preferred around 99% to achieve a higher finesse. However, 97.8% is still
decent. The reflectance of the "HR" - which could be even higher than the
measured 99.8% since the 0.02% transmission measurement was not very accurate
- might be too high to get a decent signal but could result in a very high
finesse. But at 543.5 nm, the "HR" mirror seems to be perfect.

The only thing not wonderful about these mirrors is that the planar side
isn't AR coated. (Since they were intended only for some tests, we
saved money by not having AR coating!) But, if they are slightly
tilted, hopefully, this won't be a major problem.

There are also several radii to choose from. For the first version, I
used the longest RoC which is a Melles Griot 01-LPK-01. This is a 10 mm
diameter BK7 lens with a focal length of -20 mm which has a RoC of
about 10.3 mm. (For BK7, the RoC of a plano-concave lens is -0.517 of
the focal length.) This results in an FSR of about 7.8 GHz. Note that
the FSR is c/(4*d) for the confocal cavity, one half that of the long radius
or planar SFPI cavities. See the previous section. So, these will be good
for all green HeNe lasers and longer cavity single mode green DPSS lasers
like the C315M and C532, as well as that Far East disaster described in the
section: Reconstruction of an 80 mW Green DPSSFD
Laser. However, short cavity DPSS lasers including green laser pointers,
the Uniphase uGreens, MCA based DPSS lasers, and possibly the Transverse
TIM622 will require a shorter SFPI cavity. The other sets of mirrors go
down to around a 5 mm RoC so another version may be built with a set of these.

However, note that since the gain bandwidth of Nd:YAG and Nd:YVO4
is over 150 GHz and the SHG green conversion also doubles the frequency
between modes, multimode solid state lasers may have frequencies which
greatly exceed the FSR of these medium length SFPI cavities.
Unambiguous display of their modes may require an SFPI with an FSR of more
than 300 GHz - a cavity length of 0.25 mm for the confocal configuration!
It's simply not impractical to grind and polish mirrors with very small
RoCs. The limit is about 2.5 mm for an FSR of 30 GHz.
Fortunately, what's often most important is
to confirm single or maybe dual longitudinal mode performance so a much
smaller FSR is adequate and desirable for maximum resolvance. With a
bit of care in interpretation, almost any FSR will be fine for this purpose.

The mechanical configuration is similar to the $1.00 SFPI except that the
rear mirror mount can be moved along the optical axis on threaded rods
to match the mirror distance to the RoC of the mirrors. A diagram
along the lines of the simple design of the $1.00 SFPI is shown in
Home-Built Scanning Fabry-Perot Interferometer 2.
Again, mine was constructed of cast off disk drive parts and other
miscellaneous junk. :) The first photodiode I used for this SFPI
was a $2.00 part from Digikey - which would have been my total cost if it
hadn't already been in one of my random stuff drawers. :) And, the
frame is a bit shorter since the RoC of all of these mirrors is so small.
Please see: Photo of Sam's $2.00 Scanning Fabry-Perot
Interferometer.

For the initial test, I am using the HR mirror set with an 05-LGR-151 green
HeNe laser head. Since this is a less than 0.5 mW output laser and the
sensitivity of silicon photodiodes at 543.5 nm is somewhat lower than
at 632.8 nm, detection is more difficult.

Furthermore, in order for the SFPI to be mode degenerate, the mirror spacing
really has to be quite close to the RoC for the confocal configuration.
Since these were originally lenses and not mirrors, the exact RoC is not
really known. OK, the real story is that I didn't locate the part
numbers of the lenses until after I did the initial construction and
wrote this paragraph! There are many ways to determine the actual RoC of the
mirrors. A collimated beam can be reflected from the mirror at a slight
angle. The focal point will be at a distance of one half the RoC.
Alternatively, a point source like a bare visible laser diode can be
imaged back onto itself from the mirror. Then, the RoC is the distance
to the mirror. However, any such measured RoC is only approximate. For
the SFPI to be mode degenerate, it needs to be quite precise and this can
only be determined experimentally.

The mirror alignment itself isn't super critical. It's best to have a way
of changing mirror distance without affecting alignment very much but simple
three-screws adjusters work just fine. The laser used for the alignment
should have a known spectrum if possible, preferably a single longitudinal
mode. As the correct distance is approached, the little peaks from all
the modes of the not quite confocal cavity - which may indeed be very small
or undetectable - will gradually merge into one peak whose amplitude will
increase and width will decrease dramatically.

Note that the MDI doesn't eliminate higher order transverse modes. It
only assures that they will appear at the same locations on the display
as the TEM00 modes. If the distance between the mirrors isn't close to
the RoC, there will be higher order modes at essentially random frequencies
relative to the TEM00 modes. The result will be very low fringe contrast
in the output as the PZT voltage is varied and lumps all over the place
in the display. However, as the correct distance
is approached, these will approach the TEM00 modes. Visually, if the
distance between the mirrors is moved slowly with the PZT around the
optimal distance, the output beam from from the SFPI (going to the
photodiode) will flash on and off uniformly across its entire width,
while on either side there will be concentric rings of light and dark
sweeping from center to edge or vice-versa.
It's actually quite remarkable that varying the PZT voltage by hand
(ramp turned off), the output of the SFPI can be tuned to all light or
all dark very precisely when the distance is just right.
In addition, alignment of the SFPI relative to
the laser is very easy. The reference I am using is to adjust the
the reflection from the planar surface of the front mirror to be
just below the output aperture of the laser, then adjust the position
of the beam (without changing its angle) to center the reflected blob
from the curved rear surface of the front mirror.

After some fiddling, I am able to see the modes of the
05-LGR-151, though the signal is extremely low level and the finesse
is poor. In addition, the modes appear to be somewhat distorted - possibly
due to the distance between the mirrors not being quite correct. Switching
the function generator to DC output mode and adjusting the voltage
through the modes of the HeNe laser shows a very complex transverse
mode pattern which is clearly not degenerate even when the mirror distance
is very close to optimal. I don't know if this is due to the distance
still not being perfect (commercial SFPIs are set to within a few um)
or due to poor accuracy in the spherical shape of the mirrors.
Focusing the beam improves the resolution and amplitude of the signal
somewhat or just due to the nonuniformity of the coating which results
in the reflectance decreasing from center to edge. A modest size aperture
(perhaps 1 mm) will probably help to eliminate many of the higher order
mode since they are quite spread out.

Up to this point, my conclusions were mixed. Yes, the jumbled peaks were
gone. And, alignment is definitely much less critical - once the distance of
r is found, any two of the three rear mirror mount nuts or mirror adjusters
can easily peak the output in no time flat. But, the resolution is
lower than my $1.00 SFPI - between 50 and 100 MHz, compared to better
than 25 MHz. Whlte the larger FSR means that the resolution will not
as fine for the same finesse, another factor may be the quality of the mirrors
(or lack thereof, actual specs unknown). A focusing lens (see below)
and modest size intracavity aperture will help somewhat. And a photodiode
preamp will help make alignment easier. As long as the reflections from
the various front optics don't return to the HeNe laser, the modes are
quite stable. However, very obvious instability results if a major
portion of the reflected HeNe beam hits the laser's output mirror. Then,
wild mode fluctuations appear in the SFPI display - some modes may
momentarily double in amplitude or disappear entirely. And visible power
fluctuations are also visible in the beam and interference patterns.

The next step will be to add a proper focusing lens as shown in the $2.00
SFPI diagram (there is none in the one in the photo). Presently, the
curved surface of the front mirror results in a large diverging effect
on the input beam. Using a long focal length lens helps somewhat. But
in a test using a short focal length positive lens mounted in a spring
clothspin on a micropositioner helps even more. This cancels out the
negative curvature of the front mirror and adds some additional focusing to
match the TEM00 mode of the confocal cavity. The signal amplitude increases
by at least a factor of 2 and the resolution also improves.

Eventually, I will probably construct a preamp for the photodiode to provide
an adjustable gain of up to 1,000 using a couple of op-amps. This
will greatly ease alignment since the height of the signal on the
scope on its most sensitive setting with a 10X probe now is only about
1/2 cm at best using the low power green HeNe laser. A possible design is
shown in Adjustable Gain Photodiode Preamp.
(Frequency compensation capacitors which may be needed for stability
are not shown.) The gain is variable from 0.1 to 1,000 compared to
the bare phododiode feeding a 10K ohm load. A gain of 10 would be
sufficient so this should have enough headroom for other lower output
power lasers and/or higher reflectance mirrors.

However, for now, I just replaced the 10K phododiode load resistor with
a 100K pot and substituted a 1X probe for the 10X probe. This resulted
in more than enough sensitivity even for the low power green laser while
maintaining adequate frequency response.

Finally, I installed a 9 mm focal length focusing lens as shown in the
diagram. This results in a collimated input beam coming to a focus
inside the cavity (the focal length of the lenses being used for the
mirrors is -20 mm).

And then it was perfect. :) Well not quite perfect - the finesse isn't
much better but it is quite stable, there is no evidence of unwanted
ghost frequencies, it is easy to align, and all in all, works quite well.
With careful alignment and centering of the input beam, I was even able to
achieve the situation where the FSR became c/(2*d) or 14.6 GHz. In this
case, every other mode display per sweep of the SFPI nearly disappeared
with the remaining ones almost doubling in amplitude.

The finesse is probably not as terrible as I'm implying. For my 99 percent
mirrors, the theoretical finesse is a bit over 150. So, 14.6 GHz divided
by 150 is about 100 MHz which is close to what I've measured. And, as
noted, it's quite possible the mirrors are actually somewhat less
reflective than the 99 percent being used for the finesse calculation.

This SFPI can be used to easily test most DPSS green (532 nm) CW lasers
for single frequency operation. It's easier to set up than a commercial
SFPI with a separate ramp generator/preamp box as my Wavetek function
generator is always there on top of the scope. :) The high reflectivity
of the mirrors for 532 nm turned out to not be a problem. The ~14.6 GHz
FSR is large enough to display unique modes for the C215M, C315M, C532.
While the cavities of the uGreen and LWE-142 lasers are very short and
have a higher FSR, it's still possible to detect spurious non-single
frequency operation since the extra modes will not be stable or have
a fixed relationship to the primary mode.

As a free bonus, the same SFPI can also be used for 1,5XX nm lasers by
swapping the photodiode. When I became obsessed with the desire to look
at the longitudinal modes of a Melles Griot 05-LIR-150 1,523 nm HeNe laser,
there didn't seem to be too many options. None of my other SFPIs (home-built
or commercial) would work beyond 900 nm. But then it occurred to me that
I already had this SFPI using mirrors coated for 1,540 nm. Being HR at
that wavelength (and probably close to HR at 1,523 nm), getting a signal
might be quite a challenge, but aside from the near impossibility of lining
everything up with the <1 mW beam from the IR laser, it was worth a shot.

I did have an IR photodetector for a Newport power meter, so Simply removing
the existing PD board would allow the beam to exit the back of the SFPI.
To have the most flexibility, the PD preamp from an SP-476 SFPI driver was
used (but not the HV scan output, though that could also have been used in
place of the function generator with a resistor divider to reduce the maximum
voltage). After a bit of fiddling with room lights out (any bit of fluorescent
light overwhelmed the signal and/or added 120 Hz ripple), a really messed up
display was obtained with all sorts of ringing and garbage (technical term!).
However, it was possible that this was due to the detector being designed
for a laser power meter with a smoothing capacitor or something else inside.

I had just rediscovered the use of cut-open germanium transistors as
sensors for 800 to 1,800 nm. With one of those, despite it's somewhat
low sensitivity, the modes of the 1,523 nm laser appeared in all
their glory. The finesse is only between 100 and 150, but it is more
than adequate for displaying IR HeNe laser modes. I don't do Telecom. :)
Since I now have a SP-470-03 which works from 532 nm through
at least 650 nm, that's what's used for routine laser testing and
characterization. So, I'll probably leave the IR PD permanently in
the $2 SFPI and dedicate it for IR lasers.

In 20:20 hindsight, the mechanical design of a confocal SFPI can be
considerably simpler than what I created. With careful alignment of
the mirrors during mounting and glueing (for the one on the PZT, which
has wedge), no adjustable alignment is really necessary. So, it becomes
a pair of plates on threaded rods. The front mirror would simply be
clamped or glued to the front plate and the back mirror would be glued
to the PZT, which is clamped or glued to the back plate. After initial
setup setting the precise confocal spacing, alignment is done by
X/Y (pitch/yaw) adjustments of the mounting for the SFPI head. This
can be a fancy kinematic or spherical mount, or simply a platform with
adjustable screws for feet.

About a year after building my $2 SFPI, I came across some other short
radius mirrors:

Surface 1: RoC (r) of 45 mm, R of 99.8% at 632.8 nm.

Surface 2: Planar, AR coated at 632.8 nm.

At first I thought these were for some Spectra-Physics dye laser.
But thinking about it, I'm now inclined to believe they were a HeNe laser
mirror goof. The specifications called for 43 cm RoC mirrors and someone
dropped a factor of about 10 between design and manufacturing. (My measurement
may be off by a couple of mm, so they could indeed be 45 mm mirrors.) Hey, if
NASA can goof up units, so can a laser company! How else to
explain that there were literally thousands of these available surplus
at one time. SP never sold that many dye lasers, but production runs
of thousands of HeNe laser tubes for barcode scanners at the peak of their
popularity would not have been unusual. Also, SP's dye laser pump mirrors
with short RoC mirrors tended to have the non-reflective side fine ground (not
polished and AR coated as with these). Also, the SP dye laser mirrors
I've seen have an RoC of about 50 mm, not 43 mm. Regardless of the origin,
I'm not complaining. The person I got the mirrors from insists they are HeNe
mirrors and will even send me a laser tube that uses them if he can find one.
In principle, I suppose that is possible but it would have to be a very
peculiar resonator configuration with a focal point inside. I won't
hold my breath in anticipation. :) He had been selling them on eBay
(sorry, no more available from there!) and had so many that he was using
them as decorative stones in his tropical fish tank. Transgressions like
that really need to be punished! :-)

Installing the mirrors and slightly reworking the frame to enable a 43 mm
resonator length, it was a simple matter to get this rig to work with much
better finesse. That is, after I realized two things:

The focusing lens from the $2.00 SFPI had too short a focal length
for the much longer cavity and was smearing out and reducing the
amplitude of the response.

The confocal distance was indeed 43 mm and not 38 or 40 mm as I
originally thought. At 38 mm, the SFPI initially appeared to work
but the display wasn't stable at all, mode amplitudes varied
depending on where they were on the ramp voltage, and the photodiode
signal was quite weak. Once it was adjusted at 43 mm, the display
looked very much like the one in a textbook. :)

The only problem with this SFPI for use with HeNe lasers is that the Free
Spectral Range (FSR) for the mode degenerate confocal configuration is
c/(4*d), which is only about 1.75 GHz for the 43 mm cavity. This is just
barely more than the Doppler broadened gain bandwidth of the HeNe
laser, about 1.5 GHz. So, there can be some confusion when
interpreting lasing lines on the tails of the gain curve, though this
is minor. However, a benefit is that the 1.75 GHz FSR provides nearly
the largest useful resolution by almost filling the FSR with the HeNe
laser modes.

I have a set of basic parts available for building a similar SFPI. Sorry,
it will cost more than $3 though. :) More information can be found at
Sam's Classified Page.

See W's
Scanning Fabry-Perot Interferometer Page for an SFPI using these
same mirrors (as well as others for other wavelengths). His mechanical
setup uses parts that are a bit more professional and several orders of
magnitude more expensive than mine though. Yet, he complains about
instabilities that my resonator frames constructed from recycled harddrive
parts and Home-Depot hardware don't have. :)

As with the $2 SFPI, simplification of the mounting is indeed possible.

The planar-planar (P-P) cavity is a configuration that will not support higher
order transverse modes at all. However, it is only borderline stable for the
TEM00 mode and extremely difficult to set up and align as the two mirrors
must be parallel to a very high precision
AND the input beam must be orthogonal to the input mirror to get decent
performance. The latter requirement is particularly troublesome in that
without an optical isolator, that any reflections will go directly back
to the source. Since the SFPI mirrors are nearly 100 percent reflecting
most of the time (except when in resonance), this means a nearly total
return. And most lasers get modestly to really annoyed when any of
the output beam is reflected back into their cavity, resulting in a
variety of instabilities and changes in mode structure.
However, where it is desired to either have
an FSR for which confocal mirrors are not available - which is most
FSRs without custom mirrors - or to be able to adjust the FSR for
various applications, there is no choice.

I built a P-P SFPI using a similar structure to that my others
inexpensive home-built "instruments". Unfortunately, Murphy took no
days off and ALL of these problems were present. I selected an FSR of
around 5 GHz (25 mm mirror spacing) so it would unambiguously display
modes of a two-frequency Zeeman HeNe laser like an HP-5517B. (Due to
the Zeeman splitting, these lasers can potentially have lasing modes
over a much wider bandwidth than the normal 1.6 GHz or so of the
Doppler-broadened neon gain curve. Think of a pair of neon gain
curves that are shifted by several hundred MHz with respect to
each-other, thus making the available range larger.) None of my other
SFPIs would be entirely suitable. My home-built one for 633 nm had an
FSR of only 1.7 GHz and the Spectra-Physics 470-03 (see below) has an
FSR of 2.0 GHz. A Zeeman laser could easily have lasing modes over a
bandwidth of 3 GHz or even more, especially one that has been rebuilt
improperly. Where the behavior is not what is expected, the aliasing
in a narrow range SFPI would make interpretation of what's actually
going on very difficult.

The planar mirrors used were near-HR (99.8%) at 633 nm with no AR coating
but ground with wedge so the reflections from the uncoated surface would
not interfere with the SFPI operation. But the home-built mirror adjusters
that were perfectly adequate for the confocal SFPI were barely marginal for
this one, requiring very careful tweaking for best response. And
they didn't want to remain aligned for more than a few minutes. But worst
of all, without an optical isolator, the modes of the usually well behaved
05-LHP-151 laser head being used for testing were jumping all over the
place due to the back-reflections. And there were never more than 3
modes present, generally only 2, and sometimes only a single mode.
Normally, there would be 4 or 5 modes at all times for this 5 mW laser.

During those periods where it behaved, the performance was quite acceptable
easily resolving the 05-LHP-151's longitudinal modes (spacing of 438 MHz)
with a factor of 3 or 4 to spare. This would have been more than enough
for use with the short laser tubes (mode spacing of 1 GHz or more). But the
needs for super-precise alignment and virtually unavoidable back-reflections
makes this impractical for my intended application of easily analyzing the
modes of a variety of HP/Agilent lasers and home-built equivalents.

Well, that was the idea anyhow. The rational was that rather than finding
a matched set of confocal mirrors for a high finesse SFPI which has proven
to be rather challenging (or at least expensive, see the next
section), why not build a cavity with concave mirrors to simplify alignment
but use the bore of a (defunct) HeNe laser tube to suppress higher order
modes. It works in the HeNe laser, right? Then, the exact cavity length
wouldn't be as critical, and it could actually be longer than the confocal
length (to decrease FSR). In addition, if it could be forced to operate
only in the TEM00 mode, there would be no loss of a factor of 2 in finesse
as there is with the confocal configuration.

To test this idea, an already dead (up to air) Spectra-Physics 088 HeNe laser
tube was sacrificed by removing the cathode-end (HR) mirror. Actually,
the entire end-cap assembly came apart at the glass-to-metal seal when,
after scoring the metal tip-off, a pair of pliers was used to try to break
it off. But there was no damage to the remainder of the tube including
the entire glass envelope. (This might have been a result of a hairline
crack already being present at the seal and the reason for the leak.)
So, the anode-end mirror attached to its mount, bore, tube envelope, and
centering spider was installed in my laser test jig, normally used with
one-Brewster HeNe laser tubes. This tube is supported by 4 Nylon screws
in two places to permit fine adjustment of centering and alignment. With this
scheme, by slightly loosening two pairs of Nylon screws, the tube could
be easily moved over a range of FSRs of about 400 to 500 MHz, hopefully
without totally losing alignment. A mirror with similar characteristics
to that of the SP-088 OC was glued to a PZT beeper element and attached
to the adjustable mirror mount.

Since the OC mirror is known to be properly aligned, its reflection could
be used to align the test laser (initially a Melles Griot 05-LHR-911).
Then when that was close, the function generator and scope were activated
and fine alignment of the adjustable mirror could commence.

This entire exercise turned out to be easier than I had expected but
the first results were somewhat under-whelming: I was able to obtain
a finesse of nearly a value of... 2, and just barely recognize what I
assumed to be the modes of the 05-LHR-911 laser as lumps. :)

Now, I was expecting and hoping for a finesse of 200 or 300 to be able
to resolve the split modes of HP/Agilent metrology lasers, a few MHz
apart. Clearly some more work was called for.

My initial thought was that the first problem was that the 05-LHR-911 beam
diverges as a fast enough rate that it clips the bore so diffraction losses
are very high and doesn't even create a stable intracavity mode volume. So,
the next step was to at least confirm this as one problem by first moving
the test laser closer to the SFPI to reduce the beam diameter. A
long focal length positive lens was also added to focus the laser beam
into the SFPI cavity.

These did help a bit but didn't produce any eureka moment.
Then something happened. My memory of the exact sequence of events is
somewhat fuzzy, but then the finesse jumped to a much more reasonable value.
Part of the problem was that the photodiode output wasn't terminated except
using the 1M ohm input impedance of the scope. This was both resulting in
saturation at higher light levels and seriously low pass filtering the
response. A 10K ohm resistor took care of that. However, I don't believe
this was the entire problem because I had tried terminating the signal with
no significant improvement. But then, fiddling with the alignment resulted
in a very dramatic increase in finesse and output level. So, it was probably
a combination of factors. Possibly the original response was not even due
to a direct path down the bore with a couple reflections off the side-wall
of the bore. Not that likely, but possible. The finesse is now consistently
at least 50 and likely over 100 at times. Not great, but more respectable
than 2! :)

That's the good news.

The bad news is that this scheme still has problems. For one, higher order
modes are still present. Not as many as with my original short cavity
SFPI, but enough to be annoying and confusing. Their amplitude can be
anywhere from 10 to 100 percent of the height of what I believed to be
the TEM00 modes. But the relative heights of the modes can be varied
by any change in alignment, even pressing gently on the mirror mount.
With the FSR of the SFPI being much less than the neon gain bandwidth,
interpreting what was going on became even more difficult.

I have attempted to more closely match the input beam to the mode volume
of the SFPI, but so far, this is not been very productive. And even if this
did work, requiring such painstaking setup for each test would be impractical.

In short, although not a total failure, this approach has significant
difficulties of its own, so I intend now to go back to Plan A, which is
to build a long confocal SFPI. :)

This would be a nice long confocal SFPI capable of
resolving the two lines of Zeeman-split two-frequency
HeNe lasers such as those from HP/Agilent and Excel. :) The typical
separation of the two frequencies (called F1 and F2) is between 1.6 and 4.0
MHz. So, an SFPI with a resolvance of 1-2 MHz would be required. This
will need both a combination of larger mirror spacing and decent finesse.
However, the basic design would be similar to that of my other SFPIs.

One possibility for mirrors would be the OCs from deceased low to medium power
HeNe laser tubes. The type that could be satisfactory would be 99%@633nm with
an RoC of 60 cm. For the confocal configuration, the FSR would be 125 MHz
with a finesse of about 150 producing a resolvance of about 0.83 MHz.
And the Gods of Dead Lasers know I have many of these mirrors. :)
However, one deficiency is their small diameter. Most are around 7 mm if
bare, but only 3 or 4 mm if mounted in the original mirror
mount stems, somewhat less than desired. Removing mirrors
intact is a hit or miss proposition, especially for those like Melles Griot
having a thick bead of glass frit. My success rate has been very low.
You don't want to see the results! An alternative
would be the OC mirrors from Spectra-Physics 084 tubes which have a similar
RoC and reflectance. However, they are in limited supply, though I could
probably dig up a pair. But a somewhat higher reflectance would be
even better, say 99.5% or even 99.8%, which would boost the finesse.
Now it turns out that I have found suitable mirrors in a somewhat
strange place - the HR mirrors of tube from Spectra-Physics 117A
stabilized HeNe lasers! (This is the same as the Melles Griot
05-STP-901 and Melles Griot has made the tubes for these lasers
for many years.) Unlike most modern HeNe laser tubes, these are
curved (rather than being planar) with an RoC of 60 cm. They also have
an AR coating to minimize back-reflections. In addition, they are
probably have a slightly lower reflectance than a true HR (but still
much higher than an OC) since the beam sampling
is done through the HR mirror. But I've already totally
destroyed one trying to get it off the mount - it fractured from scraping
the frit, not even particularly vigerously. And my supply is rather
limited - 3 or 4 more at most are available from dead SP-117A tubes.

But the required length of the SFPI using 60 cm RoC mirrors would be somewhat
unwieldy, so what I'd really like would be a smaller RoC to make the instrument
more compact - say 30 cm - and with a reflectance of 99.5% to 99.8%.
Unfortunately, this combination or RoC and reflectance is rather hard to
find. Melles Griot does offer some curved HR mirrors, but
they cannot guarantee that the reflectance wouldn't be so high as to be
useless for an SFPI. And while not that expensive as these things go,
they wouldn't be free or cheap enough to try out.

I also have several New Focus kinematic mounts for 1" optics. These are a
bit large, but hey, you use what you have! :) And the nice thing about these
is that they have 3 screws (rather than two adjustment screws and a ball
bearing for the pivot). So the mirror can be translated
precisely along the Z axis to fine tune the spacing. Then, all that's
required is a rigid frame. I considered using the L-bar resonator from
from a defunct Spectra-Physics 124 laser. Its length would be ideal for
use with 60 cm RoC mirrors and one of the mirrors could be installed in
the existing mount at one end. But simply disassembling that laser would be
a fair amount of work and I'd have to schedule an appointment with
the Gods of dead lasers to determine what else might be needed in the
way of chants and incantations. :)

In the end, I decided to build the smaller instrument, at least
for now. It would use a pair of 30 cm RoC OC mirrors salvaged from some
unknown 1 or 2 mW HeNe laser tubes. I have several of these, already
removed from the tubes (no further special approvals required),
have tested their reflectance, and selected a
pair with the highest, about 99.0 and 99.2 percent. (Their average
reflectance, 99.1, will be close enough for calculation purposes.)
These mirrors will result in an FSR of around 250 MHz (for the
confocal condition) and theoretical
finesse (with respect to the confocal condition) of
π*sqrt(0.991)/(1-0.991)/2=174 and a theoretical resolvance of about
1.41 MHz - sufficient to view the split modes of any
HP/Agilent two-frequency metrology laser.

I dug up a length of extruded aluminum stock to serve as a rail.
The two New Focus mounts were attached directly to it with the
relevant surfaces of the mirrors spaced exactly 30 cm apart. The
adjustment screws provide approximately a 10 mm total range in
distance to allow for a normal tolerance in RoC. My determination of
RoC is not very precise, based on the location of the focal point
of a collimated HeNe beam reflected from the mirror. So I'm assuming
that it is a nice round number of cm! The final determination would
be made by adjusting the SFPI for the mode degenerate condition.
Should the RoC turn out to be in error by more than 10 mm, the
hole by which one of the mounts is attached could easily be
elongated or moved. However, I believe the RoC is very close to 30 cm. :)

The parts went together easily with the two mounts positioned such that the
distance between the surfaces of the mirrors was 30 cm. One mount is
secured in an elongated hole just in case. :) The 99.2% mirror
was glued to a $1 PZT beeper from Digikey and the 99% mirror was sandwiched
between a pair of small hard drive platter clamping plates so it could
easily be swapped with something else if needed. All the knobs were
removed from the New Focus mounts so that a hex driver is needed for
adjustment, making inadvertent screwups less likely. (The knobs can
be easily put back on, another benefit of the New Focus mounts, but
that will probably never be needed.)

Please see: Photos of Sam's High Resolution Scanning Fabry-Perot
Interferometer 1. The frame is a recycled chassis rail. The New
Focus mirror mounts are ideal for this application since they have three
adjustment screws so that the cavity length can be fine tuned. They
also have four 8-32 tapped holes conveniently positioned to hold stuff. :)
The front mirror (lower left photo) is gently clamped between a pair of
metal plates with a nylon washer for cushioning, and then the entire
assembly is clamped to the New Focus mount with 4 screws. The back
mirror (lower middle photo) is glued to the PZT
beeper, which is clamped to the New Focus mount with 4 Nylon screws over
a plastic sheet to insulate it from the chassis. The insulation makes it
possible to drive the PZT with my SGSF1 ramp generator, which can be
configured for a 50 V p-p signal from a pair of opposite polarity outputs.
(As it turned out, this was not necessary as even the 15 or 20 V p-p
available from a function generator was more than
enough to span several FSRs.) The photodiode has an approximate active area
of 5x8 mm (because that's what I had available) plugs into a pair of
socket pins pressed into a black plastic sheet (lower right photo).
A 10K ohm load resistor provides appropriate termination as long as
the maximum transmitted optical power is less than about 0.1 mW. For
higher power lasers, either optical filters can be added to drop the
power, or a proper transimpedance (zero voltage offset input) amplifier
can be used (one of these is part of SGSF1).
Filters have the added benefit of reducing back-reflections into
the laser, which most lasers do not
like very much. Though not as much of a problem with a confocal SFPI as
with a plane-plane SFPI, they are still possible. To minimize stray
light into the photodiode, a cylindrical shield (removed for the photo)
about 2 inches in length normally surrounds the back mirror/PZT assembly.
The feet (2 at the front and 1 at the rear) are 4-40 machine screws
turned down to a point allowing for fine alignment of the overall SFPI
with respect to the laser.

Of all the SFPIs I've experimented with (both home-built and
commercial), there is no doubt that this one is by far the easiest to
align. Practically just placing it in the general vicinity of a laser
results in modes on the scope. :) The nice New Focus mirror mounts
help but their contribution is very minor. Fundamentally, there is a
wide range of alignment - both of the mirrors and of the SFPI with
respect to the laser - over which a usable signal with decent
amplitude and resolution is produced. In fact, there was a credible
mode display the first time it was switched on. The only alignment
done prior to that was to set up the laser so its beam went
approximately down the axis of the SFPI, and then adjusting the
mirrors to get a clump of reflected spots on both mirrors. With most
optical systems "long" and "easy alignment" are oxymorons. Not so
with this configuration, a benefit of the stable confocal singularity
and the ease with which the incoming beam can be aligned with the SFPI
mirrors. After some minor touch-up of the mirror alignment and
distance, the performance is approaching expectations. There's no
doubt that it is running at the confocal distance (or very close to
it). The display is quite stable with only the two modes of the
05-LHR-911 laser present, changing during mode sweep in the normal
manner. Of course with an FSR of only 250 MHz and a longitudinal mode
spacing of 883 MHz for the 05-LHR-911, the peaks don't have the normal
spacing. It is in fact 883 MHz modulo 250 MHz, so they are actually
located (relative to one of the modes) at 0 MHz, 133 MHz (883-3*250
MHz), 250 MHz, 383 MHz, etc. However, their amplitudes vary exactly
as they would with a larger FSR, but they move across the
screen really quickly due to the FSR (250 MHz) being much
smaller than the neon gain bandwidth (~1.6 GHz). The peaks are clean
and symmetric, there are no smaller ghost modes, none of the random
variations in mode heights that are usually present when the distance
is way off, or the peculiar slow mode height variations not correlated
with laser modes seen with the non-confocal barcode scanner tube SFPI
described above. It's about as close to a textbook display as possible
without being in a textbook. :) The finesse may still be
a bit lower than what calculations predict - perhaps 100 instead of
174 (with respect to the confocal FSR of 250 MHz). But even with
a finesse of 100, it should be possible to resolve the split modes of
an HP/Agilent 5517D or 5517E, and probably a 5517C. But there's still
room for improvement and fine tuning of alignment and cavity length
help somewhat. A long focus positive lens didn't have any dramatic
effect one way or the other, though while fiddling with it (stuck on a
"third hand") I stumbled on the condition of perfect alignment where
the FSR doubles - every other set of modes vanishes and the remaining
modes double in height, with the effective finesse then becoming 200
with respect to the 500 MHz FSR. But that was probably a coincidence
as it occurred again later without the lens.

What I hadn't realized initially was that the sensitivity of the
display with respect to offset from the confocal cavity length is
much lower for a longer SFPI, with more than one turn of the 80-pitch screws
being needed to produce a noticeable change in the display. Having just
fought with the 30 GHz Coherent SFPI's 5 mm cavity where 1 or 2 degrees of
rotation was significant (and difficult to do consistently), this was most
welcome. The only annoyance is the frame. Aluminum is not exactly the most
thermally stable structural material. But covering the SFPI with a
cardboard box helps. :-)

The next test was to try a Hewlett Packard 5517C, with a split frequency of
around 2.4 MHz. (I had performed my "soup can mod" to bring the split
frequency down to near the lower end of the HP-5517C range, 2.4 MHz.)
The laser was set up on an adjuatable platform with an
HP beam expander oriented backwards to
reduce the beam diameter to around 1 mm. And sure enough, once
everything was arranged on appropriate high tech wood blocks :) to get
their height approximately equal within the range of the
adjustments of the laser platform and SFPI, it was immediately obvious
that when the 5517C was locked, there was only a single peak per FSR
since it is single longitudinal mode, but that it was jagged at the top, not a
normal mode. (While the laser was warming up, two clean modes could be seen
over part of the mode sweep cycle and the jagged one being present during a
small portion, mostly by itself.) With careful adjustment of alignment
and some fine tuning of cavity length, the existence of the two Zeeman modes
was clearly visible. The peaks weren't distinct and totally separate, but a
10 to 20 percent dip could clearly be seen between them. Based on
a Matlab simulation, this indicates that the finesse is probably
quite close to 100 (relative to the 250 MHz confocal FSR).
Not surprisingly, the resolution is often, but not always,
highest just about when the optimal alignment is achieved such that the FSR
doubles and every other mode disappears (or at least gets much smaller).
Interestingly, the actual resolution of the larger and smaller modes are
not generally the same (even accounting for their difference in height). The
larger peak usually has a more pronounced dip but not always. More below.
It was fairly easy to obtain a finesse of about 125 with a dip of around
25 percent. I also tried the laser without the beam reducer (its beam is
only 3 mm in diameter), but the results were somewhat worse. Rather than
include all the boring details, suffice it to say that in the end (or at
least what passes for the end so far), the beam reducer at its narrowest
setting along with careful alignment of the laser, beam reducer, and SFPI,
as well as positioning the SFPI closer to the laser, and last but not least -
cleaning the mirrors - produced the best results with a finesse of around
160 and a dip of around 45 percent. This is very close to the theoretical
finesse of 174 shown in Simulation of Sam's High
Resolution SFPI 1 Display of HP-5517C Modes which includes plots of
more than a full FSR (400 MHz range) and a zoom-in of the Zeeman-split
peak (20 MHz range. The plots for the measured finesse of 160 are
barely distinguishable from the these except that the dip is about
5 percent less deep. The real-time display looks very similar to the
simulation except that every other peak is smaller as noted above.
However, it's just much easier to capture the results of the simulation
than to photograph a jittery expanded scope trace! Better cleaning
would probably recover the missing finesse. The mirrors must
collect dust even being vertical surfaces because I know the finesse
had been declining over the last few days. I need to make a cover. :)

Recall how I said this SFPI was easy to align? Well, that was to get
some sort of display. To actually approach the theoretical finesse required
significant effort and time. And it's still not clear what combination
of conditions actually results in the best finesse, but it always occurs
near to point of perfect alignment but not precisely at it. One set of
peaks is about 50 percent larger than the other set. The larger peaks
have the high finesse while the smaller peaks may be half of that
or even less even with the cavity length set optimally for the confocal
condition. But the finesse is actually lower at perfect
alignment when one set of peaks disappears. My speculation is that
this would be the optimal condition if the cavity were perfectly mode-matched
to the incoming laser. The finesse of that larger peak also seems marginally
higher if the cavity is a bit short compared to the confocal length by
somewhere around 0.5 mm. Getting all the way to the finesse
of 175 predicted by theory (at which point the dip should extend down to
approximately half the peak) should still be possible but may
require sacrifices to the Gods of Laser Instruments. :)
Or, as noted, it may simply be a matter of cleaner mirrors, which
would still require those sacrifices given how much I detest
cleaning mirrors. But a few nice crunchy dead HeNe laser tubes
would probably be acceptable sacrifices. :) However, there's little point
in spending yet more time removing every last molecule of contamination
from the mirrors unless the SFPI is covered to keep them
clean! Of course, it's also possible that my measurements of the mirror
reflectances weren't accurate, as hard as this may be to believe. :)
But if the reflectance of both mirrors were 0.99 or one was 0.989 and the
other was 0.91, then the theoretical finesse is only around 149 - so it's
already way past that!

The Zeeman-split modes were also not quite equal in amplitude, but that's
in the laser and due to the modified control PCB in this 5517C.
Unlike the standard one, I added a mode balance pot which was
not set correctly. It was interesting to watch the relative mode amplitudes
change as that pot was adjusted. The relative split mode heights also vary
slightly, which may be due to back-reflections even through the ND filter.
And, this high mileage laser had significant amplitude and/or frequency
ripple due either to a defective HeNe laser power supply or plasma
oscillations which were showing up in the display
and confusing the interpretation. These were eliminated by running
on a different HeNe laser power supply at a slightly higher tube current.
I'll have to fix that. :) For more on HP/Agilent metrology lasers, see the
section: Hewlett-Packard/Agilent Stabilized
HeNe Lasers.

And it turns out that this home-built SFPI actually has an even better
theoretical resolution than the fancy long Coherent SFPI that I have wanted.
The Coherent SFPI has an FSR of 300 MHz with a finesse of 200 for
a resolution of 1.5 MHz (compared to 1.41 MHz for mine). Of course
it does have the advantage of using a proper sealed low expansion
cavity with a nice adjustable mount. But then there is the
small difference in cost. :)

Next up: An SFPI with several times the resolution!
This wouldn't be as spectacular as the ultra-high resolution SFPI described
below, but would be along the lines of something you or I could build with
a bit of lucky scrounging or real money. It would have the following
specifications:

Cavity type: Spherical confocal.

Cavity length: 50 cm.

FSR: 150 MHz.

Finesse: 500.

Resolvance: 300 kHz.

All that would be required are a pair of high quality mirrors with an RoC
of 50 cm and a reflectance of 99.7 percent. An actual plot of the
two modes of an HP-5517A laser (1.65 MHz separation) made with such
an instrument can be found in High Resolution Scanning
Fabry-Perot Interferometer Display of HP-5517A Modes 1. However, this
was not made by me, as suitable mirrors have yet to materialize.

Building these things from scratch can be quite rewarding, but it does become
somewhat boring after the 15th or 20th. :) For around $100 in readily
available parts, many of the headaches can be eliminated. The following
assumes the use of "30 mm Cage" parts from Thorlabs. The parts list for
a short RoC (e.g., 43 mm/1.7 GHz) SFPI is then:

The CP02T and the SM1AD8 is used to mount the front mirror while the
CP02 is used to mount the PZT on one side and the photodiode on the other.
The CP11 is for mounting a focusing lens. The 4 cage rods allow for quick
and easy coarse adjustment of cavity and lens spacing. Rotation of the
SM1AD8 is then used for fine cavity spacing, locking in place with one of
the retaining rings. This isn't quite "plug and play" as some gluing or
drilling and tapping is required, as well as a few other odds and ends
to complete the SFPI assembly as shown in SFPI Frame
Using Thorlabs Cage Parts and Most Other Components. But the effort
will still be an order of magnitude less than building one of these from
salvaged harddrives and scrap aluminum. :) Depending on the rod length
(4 inches in this case), almost any confocal cavity can be accommodated
with the appropriate RoC mirrors and focusing lens.

To simplify initial alignment, a KC1 cage-compatible 3-screw kinematic mount
can be substituted for the CP02, but this does roughly double the cost and
increases the size and bulk. The prototype of such a rig is shown in
SFPI Head Using Thorlabs Cage
Parts with Adjustable Front Mirror. It's kind of a waste though since
once the SFPI is set up, the adjustable mount is of little value and possibly
a liability as its settings can change. However,
being able to precisely align the two mirrors so their optical axes
coincide may result in slightly better performance. An alternative to a
KC1 would be to mount the PZT on a plate with 3 springs or split washers
so that its alignment could be fine tuned. Added cost: $0.00.

Having said all that, the simpler SFPI head design really does work just fine.
The completed unit mounted on a 3-screw adjustable platform is shown in
Completed SFPI Head Using Thorlabs Cage Parts.
Coarse mirror spacing is set by moving the CP02T with the front mirror. This
can be done by first setting its position based on the known RoCs of the
mirrors, and then by maximizing the envelope of the SFPI display of a well
behaved laser like an 05-LHR-151. Fine tuning is then performed by rotating
the SM1AD8 adapter holding the front mirror using an improvised spanner made
from a large paper clip. :) A tapped 4-40 hole added on the side of the CP02T
for a soft-tipped set-screw that is snug but still allows movement would
enable smooth adjustment and lock the setting in place. An opaque cover
should be added to prevent ambient light from getting to the photodiode
and protect the mirrors from dust and other contamination. The 8-32
tapped hole in the CP02T can be used to attach the SFPI head to an
adjustable platform or multi-axis mount as in the setup, above.
A photo of an actual scan from this unit is shown in
SFPI Mode Display of Melles Griot 05-LHR-151.

Both of these are quite stable and worth every penny. ;-) The only problem
seems to be a ringing in the $1 PZT at the beginning of the scan due to
the abrupt change in direction, resulting in distortion of the display
over the first 10 percent of the scan when running at 100 Hz, with a
proportionally lower percentage at lower scan rates. This is an issue with
all of the SFPIs using the thin beeper PZTs. Using a similar PZT but with a
higher resonance frequency, a modified waveform that starts more gradually,
or applying damping material to the PZT would help. Partially
coating the back of the PZT with hardware store acrylic caulk did in fact
reduce the duration, though a lossier material would probably be better.
Of course, in the grand scheme of things, a bit of distortion over less
than 10 percent of the scan is something that is probably tolerable.
If not, simply delay the scope trigger by 1 or 2 ms. ;-)

I have also now designed a stripped down function generator especially for
driving the PZT of these SFPIs. See Sam's Scanning
Fabry-Perot Interferometer Driver 1.
This unit generates a variable frequency triangle (approximately 5 to 200 Hz)
or sawtooth (approximately 10 to 400 Hz) with a full range adjustable offset.
The output amplitude may be set from 0 to to over 25 V p-p, with
both non-inverted and inverted outputs. Where neither side of the
PZT is grounded, it may be connected between the two outputs to provide
a voltage range of up to more than 50 V p-p. The output may also
be set to DC and adjusted over the full range using the offset control for
initial setup of the SFPI. The sawtooth has a slew-rate limited falling
edge so there is no risk of damage to the PZT or excessive ringing at the
start of the scan. The adjustable amplitude implements sweep
expansion to enable examination of the fine detail of the laser spectrum.
A photodiode preamp is built in to SG-SF1
completely eliminates the need for anything beyond an oscilloscope
and +/-15 VDC power supply.

Using better op-amps than the jelly bean
LM358s might increase the maximum output voltage range slightly but
at these frequencies, won't make much difference in any other respect.
Of course, it would be trivial to modify this circuit for a different
frequency or voltage range. But, as drawn, it will cover the needs
of most SFPIs using "drum head" type PZTs, as well as the Thorlabs
SA200 and SA210.

Blank PCBs are now available for the combined PZT driver and photodiode
preamp. The PCB for SGSF1 is just under 2x2.5 inches
as shown in Photo of Sam's Scanning Fabry-Perot Driver
1. Power requirements are regulated +/-15 VDC at 50 mA. A suitable
dual DC power supply would be trivial to construct using a wall adapter
putting out 14 to 16 VAC, a pair of diodes, a pair of 1,000 uF, 25 V filter
capacitors, and 7815 and 7915 (or similar) IC regulators. SGSF1 may be
used as shown, or built into a project box with front panel controls in place
of the switch(es) and trimpots. A selector switch and fixed resistors may be
substituted for the pot to provide calibrated expansion factors (e.g., 1X,
2X, 5X, etc.). And the offset control could be a 10 turn pot.
For more info, please go to
Sam's Classified Page or
contact me via the Sci.Electronics.Repair FAQ Email
Links Page.

For displaying the longitudinal modes of some types of lasers, it is desirable
to be able to view the polarized modes separately. This would probably
be most useful for common random polarized HeNe lasers and Zeeman-split HeNe
lasers, though it could also be used with other types of lasers that are
not polarized. (These's no benefit with linearly polarized lasers.)

All that's required is to replace the normal sensor with a polarizing
beam-splitter and a pair of photodiodes (and possibly separate pre-amps).
The polarizing axes of the detector will need to be oriented to be the same
as those of the laser, but most SPFIs allow for the sensor to be rotated
in its mount. For axial Zeeman lasers producing circularly polarized outputs,
a Quarter-Wave Plate (QWP) would be required to convert to linear polarization.
A QWP is usually part of a commercial Zeeman laser tube assembly. However,
if you're rolling your own, then an external QWP will need to be added.

This is most dramatic with a fancy digital scope so the polarized modes can be
shown in living colors like red and blue in real time. :) However, any old 2
channel scope will work. Or, the SFPI can be swept at low speed if necessary
with the PD outputs fed to two channels of a PC data acquisition system.
Where the polarizing beam-splitter is wavelength sensitive, a separate unit
may be needed for each mirror set/wavelength range of interest.

As an example, for the SP-470, the normal sensor assembly can be replaced
with a short piece of 1/2" PVC pipe filed down to fit into the
SPFI housing into which a 4 or 5 mm PBS cube and inexpensive photodiodes
are mounted. The PD signals can be fed directly to two scope channels with
only 10K load resistors, or through suitable preamps depending on the laser
power involved. The one I built worked reasonably well, but there was between
5 and 10 percent crosstalk, due to a combination of the PBS cube not having
perfect separation, not being optimally oriented, or reflections off the
faces of each photodiode getting into the other channel due to their close
proximity. In fact, even the best polarizing beam-splitter cubes may have
substantial residual P-polarization in the reflected S-polarized output,
though the transmitted P-polarization is generally quite pure. (A
non-polarizing beam-splitter with pieces of sheet polarizer mounted in front
of each photodiode might actually have better separation.) So, rather
than ripping the PD assembly apart and then finding that the crosstalk was
still present even after extensive fiddling and frustration, I built a
two stage preamp which includes a means to suppress it by subtracting a
small (adjustable) amount of channel 1 from channel 2 and
vice-versa. The trans-impedance (first) stage needed to be a high
speed op-amp, a MC33077 because it was handy. With the original
jelly-bean LM358, there was serious overshoot and ringing at all
but the lowest gain settings. And with small capacitors
(e.g., 10 pF) across the feedback resistor, the
waveform still had artifacts if the amplitude was more than a few
hundred mV. While a larger (100 pF) capacitor resulted in a clean output,
the frequency response was reduced to where the SFPI resolution would
be compromised at the normal scan rate. The second (subtracting) unity
gain stage was perfectly happy with an LM358. The crosstalk cancellation
works quite well when SFPI alignment is close to optimal. Otherwise, there
may be a small phase shift between two channels making perfect cancellation
impossible.

A similar crosstalk cancellation technique may be applied elsewhere
to allow for the use of less expensive polarizing optics or where alignment
cannot be easily optimized. The cross-gain for each channel should be
set up with a pure linearly polarized beam aligned with its axis while
nulling the output of the other axis assure optimum orthogonality.

Dual Polarization SPFI Display of HeNe Laser with
Higher Order Spatial Modes shows a series of screen shots from my
antique Tektronix 465B analog scope (sorry, no color). All the peaks with
a height of more than about one screen division are the normal longitudinal
modes and they behave, well, normally. However, the smaller peaks appear
to belong to a weak non-TEM00 higher order spatial mode. This isn't
necessarily a laser where the dual mode display provides a critical
benefit, but being able to easily see how the rogue spatial modes behave
with respect to polarization is somewhat illuminating. Sorry, pun
intended. :-) While the dual polarization display doesn't help to
reveal the existence of the higher order spatial modes, it does shed
light on how they associate themselves with normal modes, or actually
run away from them. Originally (with the normal SFPI), I had assumed
that the little blips closest to a larger normal mode were of the same
polarization, not orthogonal to them, which is the actual situation.

And I couldn't resist creating a complete design including the
Sam's Dual Polarization Photodiode Preamp 1
Schematic and Sam's Dual Polarization Photodiode
Preamp Front Panel Layout. No PCB layout yet, sorry. :) This same design
can also be used for general data monitoring or capture using a data
acquisition system. For use with an SFPI, power would most conveniently
be provided by a 14 to 16 VAC wall adapter feeding the AUX PWR connector, J3.
But for other applications, the the power pins on the output connector
can be used. A PCB may be available in the future.

SG-DP1 Specifications

These assume the component values shown in the schematic:

Outputs: Channel 1 (CH1), Channel 2 (CH2), and average (AVG) or
difference (DIFF) jumper selectable. Both AVG and DIFF are available
at jumper pins (but not output connector), so may be used simultaneously.

Total transimpedance gain: 0 to greater than 25,000 V/mA.

First stage: Selectable gain of approximately 1, 10, 100,
and 1,000 V/mA.

Polarity: Photodiode may be installed to produce a positive or
negative response in photovoltaic mode or positive response in
photoconductive (back-biased) mode.

Bandwidth: Up to greater than 100 kHz using the component values
shown (on X10 gain range), limited by the response of the photodiode and
op-amps.

Offset: Adjustable from at least -15 to +15 V (subject to +/-Vaa
and op-amp), independent of gain setting (CH1 and CH2 only).

Photodiode: May be Si, GaAs, or other photodiode. Schematic shows
DB9 wired to be compatible with SP-117/A.

Op-Amps: First stage uses a high performance op-amp like an
MC33077 but the jelly bean LM358 should be acceptable for the other
lower gain stages. Higher performance devices can be substituted
where frequency response, noise, and stability are critical.

Power: Wall adapter or transformer providing 14 to 16 VAC, +/-15
VDC regulated power supply, or +/-17 VDC to +/-20 VDC unregulated power
supply (internally regulated). The current consumption is less than 50 mA.

Most common random polarized HeNe lasers produce only two sets of orthogonally
polarized longitudinal modes, generally fixed with respect to the tube
orientation. However, this is not always true and there can be modes at
arbitrary angles. One example is of a Zeeman-split laser that was intended
to be a clone of an HP-5517D. However, it used an HeNe laser tube that was
too long, resulting in a pair of low level rogue longitudinal modes in addition
to the normal Zeeman modes. In that case, the resulting modes (after the
waveplates) were linearly or elliptically polarized and orthogonal to
each-other, but oriented at 30 degrees with respect to the primary modes.
(The normal linearly polarized output modes result from circularly polarized
tube modes, but for the rogue modes to be off-axis may mean that they were
elliptically polarized tube modes.) The dual polarized mode
SFPI would have easily identified those since they would show up on both
channels and the detector assembly could then be rotated to maximize
channel separation. Where pure linearly polarized modes could not be
achieved, the degree of elliptical polarization and orientation could be
easily determined. Hmmmm, the detector assembly will need to be mounted
on a calibrated rotation stage. ;-)

Note that the success of the dual polarization detector assumes that the
SFPI itself is polarization insensitive. An SFPI has no polarizing components,
but thin film mirror coatings may have a very slight polarization anisotropy
even at normal incidence. This would not be a problem for most applications,
but where hundreds or thousands of reflections are involved as in an
interferometer, even a small polarization anisotropy in one of both
mirrors could add up and result in a polarization preference or other peculiar
behavior. So, it's worth checking the SFPI with a linearly polarized input
and confirming that the output polarization remains pure and tracks the
input as its orientation is changed. I don't know if this is ever an
issue with commercial SFPIs though since a polarization asymmetry would
also result in a response that depends on the laser polarization or
SFPI orientation even with the normal polarization insensitive detector -
something that should not generally be present.

An alternative implementation that would have no (new) issues with mirrors
would be to use the normal detector with a switchable polarization rotator
in front of the SFPI (like the LCD device in HP/Agilent lasers but possibly
faster). The polarization would be oriented horizontal and vertical
on alternate scans with appropriate scope triggering.

While I assume that every patent for SFPI technology has something
along these lines of: "Although specifically described with respect to
a single detector, anyone skilled in the Art will recognize that this approach
can be used with multiple characteristics of laser light such polarization
or wavelength each with its own detector.". Nonetheless, should a major
company read this section and decide to market a product, I DO expect
royalties. OK, right, pigs will need to fly first on both counts. ;-)

The basic setup of the SFPI hasn't changed in any significant way in over
40 years: A ramp generator, photodiode preamp, and (user supplied)
oscilloscope. While this is certainly adequate, what about alternatives
made possible by modern low cost digital technology.

One possibility would entail converting to a (PC or Mac) USB
interface with both the user controls and display being on the computer.
However, I would caution against rushing full speed into such a project as
it's extremely difficult to implement a software driven user interface
on a conventional personal computer that approaches the utility and
convenience of real knobs! But this would open up the possibility of
providing additional measurement capabilities as well as more advanced
functions like closed-loop etalon control. And the full color display
would add a touch of class. :)

Or, how about fully custom hardware? While I rather doubt that the
development of such a device could ultimately be justified based on expected
profits due to a relatively limited market, it never hurts to dream.
See Sam's Proposed Microprocessor-Based
Stand-Alone Scanning Fabry-Perot Interferometer Controller. This would
use a high resolution color LCD display with 10 soft keys and 4 soft knobs for
all user interaction. The objective with the GUI would be to have the most
commonly used functions expected in a conventional SFPI always available
without descending into endless menus. In addition, capabilities not found
in any commercial SFPI like sweep magnification (in addition to sweep
expansion) can easily be provided, which zooms in on only a
variable portion of the central area of the sweep. And when not being
used for an SFPI, your kids will love the color Etch-A-Sketch™ app,
included free for a limited time only. ;-)

A university student has sent me their Senior Project proposal for
a microcontroller-based SFPI system based on a commercial confocal
SFPI head. Stay tuned for the exciting developments.

Scientific Connections
has apparently developed (or at least developed the specifications for)
an advanced SFPI called EagleEye™ which interfaces via USB and
in addition to the normal SFPI functions, can compute linewidth well
below the resolution limit determined by the finesse and FSR of the
confocal cavity. This is done by controlling the drive voltage while
measuring the photodetector response to locate the FWHM points on
a mode. They claim a raw resolvance of 1 MHz but a computed linewidth
limit of 20 kHz. This may all be vaporware though - I have
asked about it via their Website.

Here's another possibility: The "DSO Quad™" is a pocket 4 channel digital
color storage oscilloscope which is based on open source hardware and software.
Search for "DSO Quad" and you'll get more than you ever wanted to know
about it, but the features relevant for an SFPI are 2 analog input channels
with a combined sample rate of at least 36 ks/sec and a function generator
output that can be set for triangle or sawtooth (among others) from 10 Hz
to 1 MHz. A driver for the scan voltage will be required if using an SFPI
head requiring high voltage, but perhaps not if one of mine or Thorlabs.
Add a dual channel photodiode preamp and this could represent a nice compact
instrument.

A tablet PC would be an ideal platform to implement the ultimate SFPI
controller and display without the need to design special purpose hardware
other than for the ramp driver, photodiode preamp, and their USB interface.
An optimized touch-screen-based GUI could enable all functions to be carried
out in a natural way while eliminating the costs (as well as retro appearance)
of hardware knobs and buttons! And most of the software infrastructure will
already be present allowing for natural finger gestures to perform most
common functions. For example, spread fingers to control expansion
or magnification; touch and drag to change position (centering), or pull on
selected mode peak to change preamp gain or use soft arrows.
The tablet PC screen layout could look similar to what's in
the diagram above, but with all interaction via the touch-screen and thus no
need for physical knobs and buttons. What what a great excuse to buy a
tablet PC (or better yet, have your company buy one for you!). Who could
resist? :)

Here are proposed specifications for a complete system, tentetively called
the LSA100, consisting of an SFPI head with built-in USB interface attached
to a touch screen-based tablet computer as depicted very roughly in
Sam's Proposed Tablet PC-Based Scanning Fabry-Perot
Laser Spectrum Analyzer. No other
equipment would be necessary to perform display and analysis of laser mode
structure, with export of the resulting data to other apps. The following
lists the common specifications for all versions not dependent on FSR or
mirror wavelengths.

SFPI head:

Sealed optics unit: Mirrors, PZT, front and rear irises.
Fine adjustment of mirror spacing (recessed screw). This could be
based on an existing SFPI head such as the Thorlabs SA200/210 so
that it would be compatible with a normal SFPI controller.

Swappable sensor assembly: Single PD or two PDs for dual
polarization (rotatable) or dual wavelength. The dual polarization
PD assembly would include a PBS and two photodiodes. The dual wavelength
PD assembly would include an NPBS and two dichroic filters.

While my $1 SFPI can be made to work, the choice in the types of mirrors that
are typically available surplus or from salvage are severely limited.
Alignment becomes extremely critical and an aperture is needed to suppress
non-TEM00 modes. In addition, reflections back to the laser under test may be
destabilizing. Though this is probably not a major issue with typical
HeNe lasers or green DPSS lasers with an IR-blocking filter in their output,
It could be significant for stabilized HeNe lasers and IR DPSS and other
non-frequency converted lasers.)

I lucked out for my $2 SFPI in just happening to have short
radius mirrors that could be pressed into service, but most
people wouldn't have this option.

For my $3 SFPI, I do have mirrors available but there are really only useful
for red HeNe lasers, for which they are nearly ideal.

Being able to specify the mirror radius of curvature, wavelength,
and reflectance, would greatly expand the possibilities and still
result in an instrument for under $100. That's really not too bad
considering it should have almost the same performance as a $9,999
commercial SFPI.

(From: Christoph Bollig (laserpower@gmx.net).)

Just some comments on the SFPI resonator options: The confocal configuration
has the big advantage that it can be used at an angle or an offset! Most
single-frequency lasers outputting at the fundamental (not frequency
converted) don't like it if they get reflections straight back, and especially
when those reflections are from a high reflectivity mirror and well aligned
to go back into the laser. And that's exactly what you need to do with a
plane-plane interferometer or even with most other non-confocal ones.

With the confocal interferometer, the best choice would probably be to
come in along the optical axis but with a slight offset. The
back-reflection will then be at an angle. Since such an arrangement
will need two round trips to reproduce, the second mirror can be HR
and the "transmission" will be through the same mirror as the incoming
beam, just at a different angle as shown in
Confocal Scanning Fabry-Perot Interferometer.
As you can see, there are no reflections back into the laser.

Another advantage is that since the second mirror is high reflector, no
hole is needed in the PZT. :)

We have considered different options for the mirrors for use with near-IR
lasers, but one of the more likely scenarios is to use a 50 mm RoC output
coupler from CASIX with either 98 or 94 percent reflectivity (NDO0205, $50).
see CASIX Nd
Laser Optics. These are also
available in other curvatures down to 25 mm). For the high-reflector on the
PZT one could use one of the CASIX standard HR mirrors from the
DPSS series (quite a few from
CASIX Diode
Pump Laser Optics Kits would do. For example, the DPO1301 or DPO1302
($45) (or the green laser output coupler from
Roithner,
also 50 mm radius). Or the DPO1303 (HR at both 1,064 nm
and 532 nm) which would then be useful for green DPSS lasers as well.

John Barry at Yale University builds ultra-high resolution SFPIs for
his work in laser cooling of molecules. (See, for example:
Laser
Makes Molecules Super-Cool.)
If there's a scientific instrument for laser nerds to drool over, this would
be a good candidate as it puts everything else described here to shame:

(Portions from: John Barry.)

The cavity is a simple confocal design consisting of a quartz tube,
two 1018 steel end-caps, a small ring PZT, and two mirrors (RoC of 500 mm,
and reflectivity of ~99.983% at 632.8 nm) based on the specifications
from the manufacturer at
Layertec Optics HR Mirror (Low Loss, >99.97%, 620-680 nm).
While the specifications list 99.97% reflectivity, this is at the
wavelength extremes (620 nm and 680 nm) and is lowest there. At
633 nm, the plot shows it to be at least 99.983%, and since they
are conservative, 99.99% or even higher is not out of the question.
One mirror is glued to a threaded
steel piece for rough adjustment of the cavity length. The other
mirror sits behind the PZT for fine cavity length adjustment.
See Photo of Ultra-High Resolution Scanning Fabry-Perot
Interferometer. The closeups at the bottom show the PZT with its mirror
hidden inside on the left and other mirror glued to the steel piece on the
right.

The cavity is designed to be athermal. As temperature increases, the
quartz expands and lengthens the cavity mirror spacing.
Simultaneously the 1018 steel expands relative to the ends of the
quartz where it is attached, thereby acting to decrease the mirror
spacing. By balancing these two effects and accounting for smaller
corrections (expansion of mirrors and PZT), the distance of the
cavity can be made to be largely independent of temperature.
Here are the specifications:

Cavity type: Spherical confocal.

Cavity length: 50 cm.

FSR: 150 MHz.

Theoretical finesse: 9,240.

Theoretical resolvance: 16.23 kHz.

The theoretical values are based on a mirror reflectance of 99.983% obtained
from the plot on the Layertec Web page, above. Since this is probably a lower
bound, potential performance may be much higher.

Now this isn't exactly the sort of SFPI head you attach to a pan-tilt mount
on a table top. It's normally located in a temperature-controlled environment
inside a vacuum chamber with feedback to eliminate outside sources of error.

With this spectacular finesse/resolvance, a plot of the Zeeman-split
modes of an HP-5517A laser, separated by 1.65 MHz, looks like the
SFPI display of a normal HeNe two mode laser where the modes are
several hundred MHz apart! See Ultra-High Resolution
Scanning Fabry-Perot Interferometer Display of HP-5517A Laser Modes.
The two Zeeman-split longitudinal modes (F1 and F2) are 1.65 MHz apart
with the span from 0 to 0.01 seconds being about 3.8 MHz (out of the
150 MHz FSR). The measured performance (based on careful analysis of this
plot - counting pixels on a blown up version in MS Paint!) are as follows:

Measured finesse: 9,179 (L), 8,329 (R).

Measured resolvance/linewidth: 16.34 kHz (L), 18.01 kHz (R).

The two values correspond to the widths of the left and right peaks on the
plot. There may be a 5 percent uncertainty. The smaller peaks are most
likely higher order transverse modes - evidently the length of the cavity
is not exactly confocal. The reason the spacing of those higher modes
is not constant relative to the two lowest order modes may be due
to the cavity vibrating. This is one reason this 150 MHz cavity is
no longer used. The newer design is both shorter and uses quartz that
is twice as thick.

(From: Sam.)

Here, we have a resolvance described
in terms of kHz when most other SFPIs can't even achieve the
equivalent number of MHz! While I'm fighting to get a resolvance
of a few MHz, this one does more than 100 times better. Of course, it
might cost 10,000 times as much to build and definitely lacks something
in the portability department! :-)

So, I emailed John and asked about borrying it - after all he did say they
don't use it there anymore! Within an hour, I received the tracking
number! As they say, "be careful what you wish for....". This will
now require real work. :) My intention being to see how well it performs
with an HP/Agilent 5517B laser in air (I'm not going to put everything in
a vacuum the way they do for their experiments).

Agilent 5517B laser on adjustable mount (far left): The
platform was built awhile back for HP/Agilent optical frequency comparison
experiments. For the most part, the laser will be set up to produce a
level beam, but experience with my high resolution SFPI has shown that
sometimes, final peaking is best done by adjusting the laser orientation.

Adjustable beam reducer (on front of laser): Experience has
shown that finesse increases with smaller beam diameter. And it needs
to be focused at the center of the SFPI cavity taking into consideration
the diverging lens formed by the input mirror. The beam reducer
consists of the beam expander from a 5501A laser mounted on a plate
with 5-minute Epoxy, screwed to the front of the
test laser. Since this normally accepts a beam originating from a virtual
point source somewhere near the back of the tube and collimates it, in
reverse it should be able to focus inside the SFPI cylinder even after
the diverging lens effect of the front mirror of approximately -1 m.
There is some amount of adjustment for fine tuning of focus position.
A pair of ND filters can be seen sitting on a block of wood leaning
against the beam reducer.

SFPI on adjustable mount: The bare SFPI quartz tube shown
in Photo of Ultra-High Resolution Scanning Fabry-Perot
Interferometer is installed inside an abandoned HeNe head cylinder
cushioned by pieces of bicycle inner tube and electrical tape. The head
cylinder is clamped on HeNe head mounts on standoffs attached to a piece
of wood (optical breadboard!) with three-screw adjusters. (Two are visible.)
The cavity length tweaking "tool" can be seen sitting near the left end of
the SFPI cylinder.

Photodetector (far right): This started out being a Thorlabs
DET110 feeding the SP-476 preamp, but a Thorlabs PDA55 has a better
frequency response, which helps with the high finesse when run at a
fast scan rate. However, the high finesse (or Q) of the FP cavity
ends up being as significant in limiting frequency response. More
on this below.

PZT over voltage protection: The PZT stack in the SFPI is rated
for 200 V max but the output of the SP-476 can easily exceed this even on
the low voltage range (300 V). The protection consists of a pair of NE2
neon lamps in series which will break down at ~180 V should the knobs on
the SP-476 be twiddled in such a way as to result in more than this. Since
the SP-476 uses a shunt circuit to create the ramp, even shorting the output
should not cause harm.

The SP-476 and DC power supplies for the laser can be seen behind everything
else sitting on foam blocks for vibration isolation.

I originally thought that the most difficult aspect will be initial
alignment of the SFPI to the 5517 laser to get any detectable signal.
A common SFPI has a finesse measured in the low
hundreds with mirror reflectivities of around 99 percent. Enough laser
light gets through the mirror (around 1 percent) and there is enough
scatter from imperfect mirrors, that the position of the beam can be
seen at both ends to provide a rough guide to alignment. With 99.98%
reflectivity, it might be possible to see the position of the beam
on the input mirror, but *nothing* worth writing home about will get
through the second mirror unless the FP cavity is in resonance. In
fact, nothing could be seen on the first mirror either.

However, initial alignment turned out to be relatively easy by simply setting
the X and Y position of both ends of the SFPI tube to approximately line up
with the input beam. At the front, this is done trivially using the beam
itself. For the back, the photodetector is centered on the beam and locked
in place, and it then acts as the reference. The, a small amount of searching
would produce recognizable blips.

Originally, the SFPI quartz tube was almost entirely exposed using
short pieces of head cylinder. Ahhh, a naked SFPI! :-) The result was that
the room had to be completely dark or else the ambient light (and 120 Hz hum
from the fluorescent lamps) overwhelmed the photodetector.
Since it wasn't possible to see any scatter from the mirrors anyhow, fully
enclosing the quartz tube inside a HeNe head cylinder didn't sacrifice
anything, and this totally eliminated the problem.

But the display was bouncing around due to vibrations. Both the DC power
supplies for the Agilent laser and the SP-476 (visible in the photo, above)
produce line frequency vibration. Since I don't have a vibration-damped
optical table with overhead racks for equipment, I put both of the
trouble-makers on soft foam blocks and this seemed to calm things
down considerably.

The next issue was that the display had lumps with multiple peaks. This
was due to the distance between the mirrors not satisfying the confocal
condition of the RoC of 50 cm. The input mirror is mounted on a large
threaded cylinder. Some careful adjustment in increments of 360 degrees
(since the mirror isn't quite perfectly centered or perpendicular to the
optical axis) tuned the distance to be quite close. However, due to the
backlash and slop in the machining of the threaded cylinder and the outer
cylinder into which it mates, fine adjustments were, to put it mildly,
hit or miss. Originally, I was simply moving it by hand but even after
building a tool out of a pill bottle, the consistency wasn't much better.
And with the high finesse, the confocal distance is extremely critical.
I'm contemplating adding some means of apply permanent pressure to the
threaded cylinder to both eliminate the free play and keep it from
changing position.

But then the mode spacing appeared to be around 10 MHz, not the 2.2 MHz
expected from the laser. At first I thought: "Wow, that finesse isn't
too bad...". The problem turned out to be back reflections into the
laser forcing it to be tuned offset from where it is supposed to be. I'm not
exactly sure how it comes up with a clean spacing of 10 MHz, unless it
has something to do with aliasing of multiple longitudinal modes with
the 150 MHz FSR. But by adding a pair of ND filters, the Zeeman-split
mode spacing now is correct for the 5517B laser based on the ratio of
the distance between the twin peaks and the FSR (2.3 MHz to 150 MHz).

Another problem was that the (temporal) frequency response was not high
enough for these narrow peaks to be displayed accurately when run at a
convenient repetition rate with a span of a full FSR or more. At first I
thought this was limited by the PD preamp in the SP-476 with a Thorlabs
DET110 photodiode. The rep rate*span product then needed to be reduced
by a factor of 10 or more to get a decent display. Switching to a
Thorlabs PDA55 amplified photodiode helped slightly, but then I realized
that the high finesse - or equivalently, Q-factor - of the FP cavity could
be even more significant by acting like a low pass filter. The response
can be modeled as an RC filter with a time constant equal to the cold cavity
decay time (Tcc), how long it takes for light inside the cavity
to decay to 1/e of its original value with no outside input.
Tcc = [1/(Mirror Transmission) * (Cavity Length)/c)] =
[1/(1-0.99983) * 1.67 ns] = 9.8 µs. Then the 3 dB bandwidth
is f3dB = 1/(2*π*Tcc) = ~16.24 kHz. Clearly,
the scan rate needs to be greatly reduced to get decent resolution
with a large span. Only if scanning in a narrow range (as in the plot
above) can a relatively high scan rate produce an accurate display.
Due to both vibrations and temperature variations conspiring to prevent
a stable display, the only way to do this using the oscilloscope without
some fancy locking scheme is to trigger on the actual peaks rather than
the scan ramp or blanking pulse from the HV driver.

With a fair amount of fiddling of the confocal spacing and alignment,
the finesse is now somewhere around 4,000 (!!) based on the Matlab
simulations, not quite up to what should be possible but not too shabby either.
See Display and Simulation of Ultra-High Resolution
SFPI. The multiple peaks in slightly different
positions captured while the camera "shutter" was open are probably due
to the way the scope was synced and a combination of PZT non-linearity
and vibration. It's possible that the response is still limited by the
SFPI's temporal bandwidth and slowing down the scan rate would
still result in some improvement. The scope time-base is set at roughly
0.4 ms/div which means that the pulse width shown in the photo (about
0.15 divisions) is about 60 µs representing a temporal bandwidth
within a factor of 2 of that of the SFPI/SP-476/PDA155 combination.
And further testing shows that the resolution does improve
slightly at a slower scan rate. There's something rather stramge about
a simple physical device whose frequency response is being seriously
limited by the speed of light! ;-)

I doubt the performance now is quite up to what might be possible, but it's
probably close. The three areas that might produce some improvement would be
(1) setting the confocal distance more precisely, (2) better mode-matching
of the input, and (3) putting the entire thing in a vacuum. None of these
are easy (especially the last!) or likely to happen so at this point, it's
probably as good as it's going to be! :)

But one relatively simple way to stabilize the display would be to
lock the peaks to the scan ramp. Then, a much narrower span and
higher repetition rate could be used. This could be done with not
much more than three monostables and an op-amp integrator. A
retriggerable monostable would be clocked by the PD output to produce
a pulse wider than the distance between the pair of peaks (PD Pulse).
A second monostable would be triggered by the start of the scan ramp to
produce a pulse to position the lock point at the desired location on
the scope screen (Delay Pulse), and its trailing edge would then
trigger a third monostable to produce a pulse with the same width
as the PD Pulse, called the Scan Pulse. Then, if the PD Pulse preceeded the
Scan Pulse, the integrator would be incrementally decreased, and if the
Scan Pulse preceeded the PD pulse, it would be incrementally increased.
The output of the integrator would be added into the PZT voltage. The
logical OR of the Delay Pulse and Scan Pulse would act as a gate so
that garbage during scan retrace would not confuse the locking scheme. This
probably won't handle serious vibration but should compensate for
environmental changes in temperature and pressure. Someday, maybe. :-)

Most of the following are no longer in production. The only exceptions are
some of those from Coherent, and apparently they are being phased out.
However, if you're inclined to actually buy an SFPI new, several companies
still do offer complete systems and components including
Thorlabs and
Toptika.

The SP-470 Scanning Fabry-Perot Interformeter head along with the SP-476
controller provides similar capabilities to my $2 SFPI for only an additional
$4,998. :) Actually, I don't know what the selling price was but these are
typically $5,000 or more. The SP-470 came in several flavors depending on
the wavelength range of the mirror set (450 to 550 nm or 550 to 650 nm) and
Free Spectral Range (FSR, 2 GHz or 8 GHz with 20 MHz or 40 MHz resolution).
The finesse is 200 for all versions. More info may be found under
Vintage Lasers and
Accessories Brochures and Manuals at the end of the section for
Spectra-Physics in the "High Bandwidth Scanning Interferometer Brochure".

One that I have is the SP-470-3, 550 to 650 nm with a 2 GHz FSR. This is
absolutely ideal for all common visible HeNe lasers including
the green HeNe at 543.5 nm. (There was no obvious reduction in resolution
at 543.5 nm, though I didn't do any precise measurements. And,
even for a 532 nm DPSS laser, the finesse was still at least 50.)
When I first acquired this unit, the cavity length was all
messed up so I had to set it for the confocal condition. This was done using
a low power red (632.8 nm) HeNe laser which has only 2 or 3 longitudinal
modes at most. After chasing my tail for quite awhile, I found the
sweet spot. The adjustment is by turning the mirror cell at the detector
end. Being recessed, a plastic "tool" was needed to get at it without
fear of damaging the mirror. It's spring-loaded so should stay put, but there
is no way to lock it in place. An external detector (Thorlabs DET-110 was
set up beyond the end of the SFPI head, which was on a kinematic pan-tilt
mount, and that was clamped down so it would not move relative to the HeNe
laser. Once the confocal condition was achieved, it was relatively easy
to jog the adjustment one way or the other to fine tune the cavity length.
And then it really did work like the diagrams in textbooks, and almost
as well as my $2 SFPI. :) OK, it is actually better in certain respects:
The solid massive resonator virtually eliminates any drift due to the short
term effect of temperature on the cavity length and also results in much
reduced sensitivity to vibration.

In fact, it appears as though the resolution may actually be much better
than the 20 MHz listed in the specs.

For a simple display of the modes of a HeNe laser, this high resolution
really doesn't matter and may actually be a distraction. I need to find
a laser that will do it justice!

However, the 2 GHz FSR is too small to display the modes of a Zeeman-split
HeNe laser without aliasing. When an axial magnetic field is applied to
a HeNe laser tube, the neon gain curve splits into two similar curves,
one shifted up and the other shifted down by several hundred MHz. Thus,
the effective split gain curve can be much wider than the nominal 1.6 GHz
or so of the normal HeNe laser. Only when the alignment between the SFPI
and laser is essentially perfect and the FSR doubles is the display
unambiguous.

The destabilizing effect of any back-reflections from the SFPI into the
laser is also very evident as random noise superimposed on the mode
display. So, either an optical isolator must be used, or the beam aimed
at an angle so the none of the reflection of the beam enters the laser
aperture.

The SP-470 allows for interchangeable mirrors and detector, unlike the
SP-450 where everything is fixed. There were also optional lenses, polarizers,
and apertures that can be screwed into the front of these SFPI heads. The
lenses can improve the resolvance under some conditions, but not always.
The aperture does generally help and also makes alignment even easier, at
the expense of signal level.

As a practical matter, swapping mirrors still requires precise adjustment
of their spacing, so it's not quite as easy as described in the SP brochure.
So, it's best to have a separate SFPI head for each wavelength range.

I've also tested a 470-02, 450 to 550 nm, 8 GHz FSR, ideal for most of the
common wavelengths of the argon ion laser as well a the 532 nm DPSS laser.

The sensor is just a photodiode with a lens to focus the center 3 mm or
so of the transmitted beam onto the detector. For a narrow beam like that
from a HeNe laser, almost any photodiode will work. However, performance
does seem to be better with the lensed photodiode compared to a cheap
non-lensed one for large diameter beams, perhaps simply because the
capture area of the lensed PD is smaller. I'm not sure exactly what
else, if anything, is inside the detector housing beside the PD socket.
What I believe to be a genuine SP detector measures 1.5K ohms across the
PD socket, but no other components are shown in the SP-476 schematic. And,
the 1.5K across the PD results in power line frequency hum, which
is particularly annoying on the more sensitive ranges. This is apparently due
to ground loops inside the SP-476 as rearranging the wiring inside the
supposedly shielded PD preamp section affects it. My home-built detectors
using cheap photodiodes mounted inside filed-down 1/2" PCV pipe couplings
work fine without the resistor and do not have noticeable
hum. (Where the resistor is present, the amplitude of the hum can be
reduced by about 75 percent by removing the black ground wire between
the PD Input BNC connector and the PD preamp PCB, and rearranging some
of the cables. But I wasn't able to get it to go away. I don't believe
there is any fault in this SP-476. It's just not built using good
analog design practices. Perhaps there's an ECO for later versions.
It might also be possible to remove the resistor inside the detector
housing, though that appears difficult.)

The SP-450 Scanning Fabry-Perot Interferometer head along with a suitable
controller has similar performance to the SP-470 but is in a smaller
package with a single permanently attached cable for PZT drive and
photodiode output. It terminates in a strange plug that doesn't mate
with the SP-476. There is no mention of that in the SP documentation
that I've seen but an adapter cable to BNCs apparently exists.
I simply cut off the plug and installed a pair of well
marked coaxes for PZT and PD. The pins are as follows:

PD is simply a silicon photodiode with no other circuitry. Normally, PD
Cathode would be grounded with PD Anode being the signal input. The polarity
for HV has frequency increasing to the right on the display (PZT shrinks
with voltage). Reverse to have wavelength increasing to the right.

Like the SP-470, the SP-450 is also a confocal design but with a fixed
approximately 1 mm hole on the input side. But this makes it very
easy to use. Just about any beam that enters the hole roughly
on-axis will result in a decent display with at most fine tuning
of alignment needed for it to be perfect.

Access to the internal components is remarkably straightforward.
After removing the rear cover and cable (3 itty-bitty set-screws), use
a thin tool to carefully pry out the large red O-ring. The entire
PZT/optics assembly can then be pulled free. It's only anchored with
a similar large red O-ring at the front. There may be a focusing lens
glued to the front optics holder. The PZT HV+ (white) wire is fastened with a
set-screw at the front whose head is possibly blocked by the focusing lens.
It's real easy for this wire to break off since the set-screw tends
to mangle the strands. I installed a larger solid wire so the
set-screw could be tightened more securely and then soldered to that,
with heatshrink for protection over the splice. The PZT HV- (green) wire is
secured with a screw into the rear of the PZT/optics assembly.

The cavity should never need adjustment but if someone before you
decided the outer ring at the rear was loose and screwed it down tight,
I believe that's what does it. :) (The inner ring secures the mirror.)
And while the description of the SP-450 didin't offer the option of
swapping mirror sets, it really should be possible.

While a low speed function generator with a maximum output of 20 to 40 V p-p
will work with my PZT beeper-based SFPIs, most commercial instruments like
the SP-470 described above require 100 V or more to provide enough sweep span.
The SP-476 Scanning Fabry-Perot Interferometer Driver is basically a dedicated
high voltage ramp generator designed specifically for this purpose.
It produces a variable frequency sawtooth ramp with selectable and adjustable
amplitude and offset. The maximum output is switch selectable between 300
and 1,000 V, which really specifies the approximate maximum range including
the centering or offset voltage, the actual p-p output of the sawtooth
is somewhat less than these values). A photodiode preamp with 5 gain
ranges, as well as a temperature controller (for SFPI setups with thermal
control capability) are also included.

Note that the SP-481 and SP-481A Dye Laser Etalon Controllers have
most of the functions of an SP-476 and some additional ones including
a set of slow speed ranges and a front panel temperature control.
(I don't know what the difference is between the SP-481 and SP-481A.) They
have a bunch more stuff as well for the dye laser etalon control application
including a separate HV output for the cavity. (Either the etalon or
cavity HV output may be used for the sweep, but not both at the same time.)
None of this interferes with its use for an SFPI. The only relevant
difference seems to be that the SP-481/A lacks a selection for 300 V
or 1,000 V maximum output - it's always 1,000 V. But since there is
a knob to adjust the amplitude from 0 to maximum anyhow, that's no
great loss as long as care is taken not to exceed the voltage rating
of your PZT.

Both the SP-476 and SP-481/A show up on eBay quite frequently, generally going
for less than $200. And they provide more features than most other SFPI
drivers including adjustable outputs for blanking/scope trigger and (scaled)
ramp, a heater controller, and the ability to be used as a high voltage
amplifier. Although they are quite old, except for the power
transformer which is custom (and identical for both models), the circuitry
isn't very complex and uses common readily available components should
repair be needed:

The only thing that is frequently problematic (by design and age)
is the stack of three 500 V, 10 uF electrolytic capacitors in the high
voltage power supply. That rating of 500 V is marginal since the total
voltage across them is very near 1,500 V - or perhaps even a bit more
depending on the AC line voltage. It's even more problematic if they
were originally 450 V caps (as apparently some were), or were replaced
with 450 V caps because 500 V caps were hard to find! Although there
are equalizing resistors across the caps, that's still running too
close to their rated voltage for my tastes! Replacing the original
three 10 uF, 500 V caps with four 20 uF, 450 V caps (and an additional
equalizing resistor) would be a good idea. The PCB layout easily
permits this, though modern caps are more commonly of the radial type
that stand up. But there's plenty of space. That is what I do if any
problems are detected, usually due to unequal leakage in old capacitors
resulting in a gross voltage imbalance across the caps.

A failure I've seen was a shorted power transformer.
I replaced that with a pair of more standard transformers - a 25.6 VRMS
center-tapped unit for the +/-15 VDC supply and a 750 VRMS unit from
a linear HeNe laser power supply for the high voltage. That 750 V is
somewhat low, so the 1,000 V setting may only go to 700 V or so, but
that's adequate for most applications. The two small transformers easily
fit in the available space.

Most common in this older equipment is the need to clean the pots and
switches. Simply working the pots back and forth may be sufficient to
clean them. Else, use a spray cleaner.
The grease in the (photodiode preamp) Vertical Gain and (scan)
Dispersion selector switches may also be gummed up making them very
hard to turn. The proper remedy requires removing the knobs and then the
snap-rings holding their shafts in place, and pushing the shafts out
(after freeing up approriate hardware on the switches themselves)
for thorough cleaning and lubrication with a light grease. DO NOT
just spray in WD-40 as it is unlikely to work but will definitely make
a mess!!! :) (In fact, NEVER use WD-40 for lubrication.)

I've blown the LM308 in the first stage of the PD preamp by connecting
the input to a HV output (don't ask!). It's possible to replace that
hard-to-find op-amp with another general purpose part. Using an LF357
(8 pins instead of 14) requires its legs to be twisted into very uncomfortable
positions and a jumper or two, but it works fine.

However, one nice thing about the SP-476 (and presumably the SP-481/A as
well) is that since the high voltage driver is essentially a voltage
controlled shunt regulator, even a continuous short circuit of the output
to ground will do no harm, apparently unlike in many other designs! :)

Burleigh, since taken over by EXFO,
manufactured a variety of optical instruments including wavemeters and
interferometers. (Though as of 2010, they have discontinued this product
line.)

The SA Plus is a system similar to the Spectra-Physics 476 controller
with 450 or 470 SFPI head, but of more modern construction and some
nice features compared to those vintage instruments. A brochure/spec sheet
can be found at
Burleigh/EXFO
SA Plus Laser Spectrum Analyzer Brochure and my backup at
Sam's Copy of
Burleigh/EXFO SA Plus Laser Spectrum Analyzer Brochure. The same head is
used for all versions but the FSR can be either 2 GHz or 8 Ghz, determined by
the mirrors and mirror holders. Mirror sets are available covering
wavelengths from 550 nm to 1,800 nm, with a finesse of either 200 or 300
depending on the wavelength range, higher for IR.

The standard mount for the SA Plus SFPI head has both pan and tilt, and X-Y
adjustments. The X-Y greatly simplifies setup as the laser then doesn't
need to be on an adjustable platform. The only problem is that the mount
is quite HUGE!

The cavity length can be easily fine tuned and then locked in position without
going inside, unlike the SP-470 which requires removing the photodiode and
using a tool to turn the mirror holder, then replacing the PD and checking if
the adjustment helped or hurt. Or, mount the PD externally and angle a tool
inside to turn the holder without scratching the mirror. There's also no
way to lock the mirrors in the SP-470 against vibrations messing up the
distance. (The SP-450 is adjusted at the factory and cannot be changed.)

The ramp generator doesn't have as many bells and whistles as the SP-476 but
it is adequate and also adds a nifty little bar-graph display to show
approximately the amplitude location of the ramp and relative to the maximum
voltage swing available.

The photodiode preamp is a separate little box powered by a pair of 9 V
batteries. The PD itself slips into the back of the SFPI head and is held
in place by a pair of magnets.

The SA Plus I tested has the mirror set for 550 to 650 nm. At least I think
it does since there were no markings on the mirror holders and I wasn't about
to remove the actual mirror glass to check them. The mirrors had that
silvery broad-band appearance in reflection and deep purple in transmission.

It works well at 633 nm and 543.5 nm, and probably even at 532 nm though I
didn't do a complete test there. I believe it will also be useful well
beyond 650 nm. Setup is very easy once I mounted the SFPI head assembly
on a Newport post holder screwed to a wooden plank. :) It took under a
minute from a totally misadjusted condition to find and fine tune the
cavity length for optimal confocal response. (I, of course, had
disassembled it to see what was inside and try to determine the
part numbers on the mirrors!)

Although the controller only has a single-turn pot for centering, that seems
to be enough. The photodiode preamp saturates at around 1 mW in each mode,
but that could be partially because the batteries were somewhat weak at
7.5 V instead of 9 V. :) An ND filter takes care of that and also helps
to reduce backreflections to the laser.

I also have the head (only) from what is probably an earlier version - perhaps
the "SA" without the "Plus". :) The manufacturer's sticker had been removed
so I do not know the precise model. It has a holder with a threaded lock-ring
for the back mirror and a plate with the mirror glued into it for the front
mirror. But it is otherwise similar with a large locking ring and fits a
standard 2 inch mount.

The mirrors in mine are for an IR wavelength range (as yet to be determined),
but it was fairly easy to replace them with 633 nm 1.7 GHz FSR high finesse
mirrors. The back mirror dropped right in being the same size as the
Burleigh mirror. But rather than attempting to remove the Burleigh mirror
from it's glue and possibly ruining it, a quick and dirty adapter plate
was fashioned out of the end-cap from a dead 3/4" diameter HeNe laser tube.
This also enabled the cavity to be lengthened by approximately 0.2 inches
to accommodate the difference in FSR (1.7 versus 2.0 GHz for the Burleigh
mirrors). The 633 nm mirrors worked quite well. It was
easy to locate the optimal spacing and lock it in place and the achievable
finesse was quite decent, tested using an Agilent 5517 laser with a ~2 mm beam.
However, the "sweet spot" for alignment was quite small possibly indicating
that the two mirrors are not quite aligned with their optical axes coincident.
So, I will probably have a proper adapter plate machined eventually, and
at the very least, it will look a lot spiffier compared to the re-purposed
HeNe laser tube end-cap!

These are another line of Scanning Fabry-Perot Interferometers (SFPIs)
similar to the instruments from Spectra-Physics and Burleigh, although
Coherent calls it a "Laser Spectrum Analyzer". These were originally
developed by Tropel, which then becaime part of Coherent (for this as
well as many other products). But unlike the other SFPIs I've come
across, this ones from Coherent were a current product (as of 2010).
There were four versions with FSRs of 300 MHz,
1.5 GHz, 7.5 GHz, and 30 GHz. I suppose HeNe lasers are no longer of
high priority to be analyzed as none of these FSRs is really optimal
for a 1.6 GHz gain bandwidth! Except
for the 300 MHz FSR, they all use the same body (spacer tube and lens
assembly, part number 33-2492) so only the mirror sets differ. The one
with a 300 MHz FSR has a body that looks more like a HeNe laser tube
being about 14.5 inches long (part number 33-2502). There are 14 mirror
sets available covering wavelengths from 337 nm to 1,625 nm (except for the
30 GHz FSR version which lacks the 5 shortest wavelength rangess.) But there
are some gaps in the IR wavelength coverage. (This may be more a matter
of specifications than anything else as it's likely that mirrors on either
side would still have decent performance in the gaps.) The finesse is
spec'd at 200 for all except the 30 GHz FSR,
for which it is only 100. If you'd like to order one from Coherent
and have a working time machine the price in 2010 was around $6,500
with the ramp driver. :-) Now (2012) Coherent has phased out the
entire product line. Go to Coherent
and search for "laser spectrum analyzer" and you'll be able to confirm
the bad news. I have saved the glossy product brochure at
Coherent
Laser Spectrum Analyser System. Since this is an obsolete product,
I'm hoping Coherent won't mind. :)

The short SFPI heads have an adjustable focusing lens in front which enables
the focal point to be optimally positioned in the center of the cavity. The
photodiode on a bayonet mount (looks like an oversized BNC)
which makes it more secure with alignment that is more precise
and consistent. The mount that comes with the SFPI head uses a gimbal
design which means that the head pivots about a common point near
the center of the mount. (A kinematic mount pivots about a point
near one corner. As a practical matter, this doens't really
matter very much for an SFPI.) The mount has pan and tilt adjustments
and is on a post that slides into
a massive base, adjustable in height. However, unlike the Burleigh SA
mount, there is no side-to-side adjustment. Fine X and Y adjustment
would be highly desirable, especially for the more finicky heads with
the 30 GHz FSR (more below). The 300 MHz version fits the same mount with an
adapter ring (since it's narrower over most of its length), but
it's not at all clear if pan and tilt with its axis near the center
of the cavity is really optimal for ease of adjustment with this
long head. The ramp driver has a maximum output
of 250 V which is sufficient for viewing 2 FSRs, but not with a lot
of margin. It appears generally similar to the
one for the Burleigh SA, but has the PD preamp built in.

The first Coherent SFPI head I acquired had the nice mount, but no driver, so
I'am using it with an SP-476 driver and that works fine.
However, since Coherent specs the maximum voltage to be 500 V, the
1,000 V setting of the SP-476 should not be used;
the 300 V setting is enough for much more than 1 FSR.
This SFPI head has an FSR of 30 GHz with a wavelength range of
550 to 650 nm. That might be useful for some types of diode lasers but
is certainly far from ideal even for short HeNe lasers with a large mode
spacing.

With the mirrors having a Radius of Curvature (RoC) of only about 2.5 mm
(1/10th inch) spaced an equal distance apart, alignment to the laser even
with the confocal cavity becomes much more tricky. The person who sent
it to me couldn't get any response at all from the photodiode, and at
first, I had the same problem. But finally, after very careful alignment,
it started to behave more like the other confocal SFPIs I've tested, except
that the finesse is poor. With a spec'dfinesse of only 100, the resolvance
under ideal conditions (adjustment, alignment, and beam focusing) is only
about 300 Mhz. So far, I've only been able to achieve a finesse of around
50 (resolvance of 600 MHz) which can barely
resolve the longitudinal modes of a Melles Griot 05-LHR-911 HeNe laser
(mode spacing of 883 MHz). I have not yet found the cause.
It is very likely at least in part due to the beam diameter being
too large, as well as the mirror spacing
not being precise enough, which seems quite likely since the
previous owner may have attempted to adjust it after not being able
to obtain a signal. And it is very fincky!
But there could conceivably be damage
to the mirrors as a result of abuse it may have been subjected to in
its previous life. The specs may simply be overly optimistic for
real World conditions.

Disassembly for adjustment or removal of the mirrors is supposed to require
special optical spanner wrenches so it's more difficult to do anything inside
(or mess it up!) than with the Spectra-Physics or Burleigh SFPI heads. But
mirror sets are intended to be replaceable by the user, so it shouldn't
be that bad. :) Since my Coherent tool set hasn't arrived, at first I
was using an improvised pair of filed-down needle-nose pliers, but this
required removing the photodiode assembly and there wasn't enough clearance
to mount it externally since it would have to be relatively close due to
the small RoC of the mirrors and divergence of the transmitted beam.
So, observing the display in real-time was not
practical. But this enabled the rear mirror cell to be removed for
inspection. Both mirrors appear fine. It's amazing how small they
are - a 2.5 mm RoC and nearly a complete concave hemisphere. How do
they even grind the substrate for a mirror like that?
Finally, threatening the SFPI with a dental pick did the trick.
Angling the dental pick into one of the holes in the adjustment ring
with an external photodiode permitted very fine movement while
observing the SFPI display. It is still VERY finicky.
Even with careful adjustment of cavity length - which
is still a pain - and perhaps a bit of fudging of the data, the
finesse is close to the spec'd value of 100 based on the FWHM, though
the lower portion of each peak is more spread out than would be
expected based on the textbook plots, and there are still some
artifacts in the display. Then again, perhaps all those assumptions
made in calculating finesse simply don't work very well with a finite
beam size and such a small mirror RoC! Something about serious spherical
abberation conspiring to mess it up. Perhaps, the beam really has to be
a smaller diameter or better collimated or something. And various
other things conspire to make this more difficult than it could
have been with better design and more precise machining. The
problem with the adjustment is that the
required setting must be accurate to well under +/-1 degree of rotation,
and even a slight change affects other aspects of alignment due to
tolerances in the machining, fit, and polish. :) For example, the
two very tiny mirrors are held inside long tubes that protrude from
mounts on screw threads at opposite ends
of the spacing tube. Expecting their focii to line up within a small
fraction of 1 mm is asking a lot. And, there is no
backing spring - apparently the special sticky grease is supposed
to keep everything in line. So, it's not clear how stable
this is over time - just a lot of fine thread area and grease.
Too bad they didin't include at least a set-screw for locking.

I don't really have a good use for a 30 GHz, 550 to 650 nm SFPI, though
it's quite possible one could turn up. However,
the Coherent model I would really have liked would be their part
number 33-6305: 300 MHz FSR, 550 to 650 nm.
Then I wouldn't have had to build my own SFPI to resolve the split
line of two-frequency HeNe lasers from HP/Agilent and Excel. However,
Coherent may not have offered this particular combination anymore
since there was no link to it on their Web site (even as of 2010).
But I would have settled for a used one. Perhaps, Coherent
might have accepted a trade. :) (As it turned out, I did end up building
my own with an FSR of 250 MHz and finesse exceeding 150. See
the section: Sam's High Resolution Scanning
Fabry-Perot Interferometer.)

Now here's the peculiar effect of the week: I wanted to put a neutral density
filter in front of the laser to reduce the maximum possible intensity of any
back-reflections. This is desirable to minimize the chance of laser
instabiliity that can result from mutually coherent light re-entering its
cavity. The transmitted power would be the input power times
the filter's transmission coefficient, so the amplitude of the display
would be reduced, but there is plenty of gain! However, any back-reflections
would be reduced by the square of the transmission coefficient, even
if there was 100 percent reflection. For example, with a filter coefficient
of 0.25, the transmitted power would be 25 percent of the laser power,
and the maximum reflected power would be no more than 6.25 percent.
I tried several filters that are basically amber-colored glass. Three
out of four behaved as expected: The optical power reaching the SFPI had
the expected value and amplitude of the display was reduced. Some
adjustment of the SFPI alignment was required to optimize the display
if the glass plate was at an angle, but the resulting
amplitude was very close to what would be expected based on the
transmission coefficient of the filter. However, the forth piece
of glass behaved, well, strangely. While it reduced the power as
measured by eye and with a laser power meter by the expected amount
(75 percent), the SPFI display disappeared entirely regardless of
the orientation of the filter or adjustment of the SPFI alignment
for an amplitude of exactly zero or 0.0000000 on the 'scope even with
the gain controls set at maximum! Not even any tiny bumps. In addition,
under normal conditions when optimally aligned, the back-reflection from
this SFPI is a large more or less uniform disk of light. That was
totally absent as well.

And to add to the strangeness, there was no similar effect using the
same laser and filter with a Spectra-Physics 8 GHz SFPI.

It turned out that the cause was very simple: The one problematic filter
has a rather large wedge - probably 1 or 2 degrees. The others have
little or no wedge. It's not enough to detectably divert the beam but
must be tilting the wavefront so the required interference cancels out.
And in fact with the wedge producing a deflection horizontally, careful
realignment including repositioning the SFPI head horizontally finally
was able to restore the display to normal. The extremely small cavity
of the 30 GHz SPFI is much more finicky about (laser) alignment in general
so it must be much more sensitive to the wavefront as well.
Interesting..... :)

As noted, these started out as Tropel so not surprisingly, the construction
of the Coherent versions is almost identical. While they look very similar
and the heads do fit the smae mount, there are some subtle differences.
At the very least, the thread diameter for mounting the photodiode assembly
differs by just enough that they are not interchangeable.
Go figure. :) I acquired a Tropel 240 with the mirror set for
green/blue lasers, which is probably equivalent to Coherent part number
33-6206 which has an FSR of 1.5 GHz. And of course the first thing I
did was to attempt to confirm that the confocal mirror spacing was optimal.
It probably was and so this became a BIG mistake. The special Tropel grease
had congealed over the eons so while the mirror barrel initially seemed to be
movable, it jammed solid and required total disassembly to be able to apply
enough torque to free it - and that was almost impossible. I nearly broke the
official Coherent mirror spacing adjustment tool in the process and had
to use filed down snap-ring pliers to rotate the mirror barrel with the
threaded metal flange of the inner cylinder clamped in a (cushioned) vice!
However, there were two benefits to this: Primarily, it
allowed the internal construction to be documented. :) See
Tropel Model 240 Scanning Fabry Perot
Interferometer Head Components. The only parts not shown are
the 4 screws that secure the BNC connector. Disassembly (and reassembly)
is straightforward except that the BNC connector needs to be
unsoldered from short stubs of the red and black wires before the inner
assembly can be removed. That's not so bad. Going the other way is
the pain. :( :) The Coherent photodetector assembly is shown; I do not
have one from Tropel but assume they looks similar even if the thread size
differs by enough to be annoying. And now the mirror spacing adjustment
is smooth as silk. I didn't have the special Tropel or Coherent grease,
so I used the tiniest bit of high vacuum grease. It may not stay put
quite as well but I can add a dab of 5 Minute Epoxy (removable should
the need arise) to prevent any drift.

Another short Coherent head I acquired had 30 GHz FSR mirrors for 900 to
1,070 nm. This would be useful for Nd:YAG and other similar lasers, but
they are kind of boring. :) So, it occurred to me that with small
adapter rings, the mirrors I provide in my SFPI kit for 633 nm could
be made to fit the Coherent SFPI head. The mirrors would need to poke
slightly beyond the normal mounting surface since their 43 mm RoC is
7 mm shorter than the 1.5 GHz FSR of the SFPI body. (The 7.5 GHz
and 30 GHz Coherent/Tropel mirrors do something similar.) At first,
I was going to cobble something together but in the end decided to
have them professionally machined. The mirrors are glued to the
adapter rings and then they are easily installed like the
Coherent/Tropel mirrors. And their performance is actually
quite phenomenal. For a 6 mm diameter beam
from a Zygo 7701 laser, the finesse exceeds 350; for a 3 mm beam, it is
over 550, and probably slightly higher for a smaller beam! (Theory predicts
it can exceed 600 based on mesaured mirror reflectances.)

The performance at 633 nm is considerably better than that of
the standard Coherent/Tropel mirrors.
The two-frequencies 20 MHz apart are resolved as nearly independent peaks.
See Scanning Fabry-Perot Interferometer Display and
Simulation of Zygo 7701 Laser Spectrum. The photo on the left is the
unretouched screen shot with a span of about 2 FSRs. The photo in the middle
is one of the twin peaks expanded by a factor of 10 on the scope. The
plot on the right is a Matlab simulation of a Zygo spectrum with an
SFPI finesse of 550.

Tests using a REO tunable HeNe laser show a finesse of between
400 and 450 at 604/612 nm and between 250 and 300 at 594 nm. I
need to test at longer wavelengths but don't have a convenient
tunable laser for that. If the reflectance function is
symmetric, the finesse at 670 nm should be in the 250 to 300 range.
However, since the coatings of HeNe laser mirrors are often designed
with a reflectivity fumction that is a "cliff" with respect to
wavelength, it's possible that the useful range above 633 nm may
not extend that far.

I now have a Coherent model 216 - the nice long one with an FSR of
300 MHz. (The "216" designation is a carryover from Tropel and seems
to have zero correlation with the more recent Coherent part numbers.)
Construction is generally similar to the shorter Coherent (and Tropel 240)
SFPIs with mirror mounts glued to a PZT spacer tube. For the long SFPI,
the PZT is the same length, but a glass cylinder extends it the required
distance. I say "glass" but it may be Zerodur or something like that.
The SFPI cavity is sandwiched between the front and back end-plates, with
only a rubber O-ring as cushioning at the front (to allow for the PZT
to do its thing). The end-plates have aluminum-on-aluminum threads, which
meant that I once again had to fight with metal-lock. But this did allow
me to inspect the inside. Someone may have repaired this unit at some
point in the past as the glue jobs on the PZT look suspect and the rear
mirror mount actually came off the glass tube, but some 5 minute Epoxy
solved that. :)

Unfortunately, the wavelength range of the mirror set was not labeled.
They have a slightly bluish tint in reflection and a slightly yellowish
appearance in transmission.

(The IDs are my arbitrary designation.
All of these were available for the 1.5 GHz, 7.5 GHz, and 300 MHz FSR heads.
The 30 GHz FSR version lacked the UV/violet options, IDs 1 through 5.
More details can be found in the product brochure at
Coherent
Laser Spectrum Analyser System.)

IDs 6 and 7 were ruled out both by their appearance and by testing
with HeNe red and DPSS green lasers. The yellowish tint in transmission
means that the wavelength for peak reflection must be in the deep blue or
3 times that wavelength in the mid-IR.

Having initially concluded (perhaps incorrectly) that the original mirrors
weren't likely to be that useful, even once their wavelength range could be
determined, I decided to installed mirrors of my own, at least as a test.
Fortunately, swapping mirrors in all of the Tropel/Coherent SFPI heads
is relatively easy and low risk, each being held in by a threaded ring with a
rubber O-ring for cushioning. The required RoC is 25 cm and the mounts have a
diameter of 12 mm diameter. I did have some HeNe mirrors that I thought had
an RoC of 25 cm, and they were the right size so I popped them in. But it
turned out that their RoC was actually 30 cm, so the results were, well, a bit
strange. They did work but with optimal adjustment of the SFPI cavity, the
display had an effective FSR of 60 MHz (1/5th of the expected 300 MHz), but
with a finesse referenced to 300 MHz. And the reflectance of these mirrors
is about 98.5 percent - somewhat lower than optimal. Thus, the resolvance
was not very good. But nonetheless, it could be seen that a Zeeman-split
HP-5517C laser produced a pair of wavelengths, though their separation could
not be measured. (This was partly due to not attempting to optimally
mode match the widely diverging beam from an HP laser without the normal
collimator to the SFPI. However, this did prove that the SPFI was
operational. So the finesse wasn't actually too terrible and in fact,
such a system could still be useful to confirm that a laser is
single longitudinal mode (SLM). Although the multiple peaks weren't
as consistent in amplitude as they would be with the proper mirrors,
non-SLM behavior would still be apparent. But this wasn't really what
I Wanted. One option would be to make adapters so that standard 6 mm
HeNe mirrors can be dropped in, of which I probably have several
suitable candidates with a somewhat higher reflectaace, but perhaps
not enough of an improvement to warrent the effort.

Later, I did some more testing of the original mirrors. They passed
405 nm from a diode laser like a sieve so that eliminated IDs 4 and 5
(above). Testing with a 1,064 nm laser tossed ID 11 and 12.
Based on the appearance, the most likely mirrors would be IDs 13 or 14.
Finally, I dug up a Lightwave Electronics model 120-02 laser that operates
at 1,319 nm. Bingo! The mirrors appear to have very low tranmission
and high reflectance at 1,319 nm. Using an IR detector card, the transmission
was essentially zero. The next step was to reinstall these
mirrors and test the SPFI in its original configuration, except using one
of my cut open germanium transistor photodetectors (since the original
sensor did not come with the 216).

I expected it to be fairly easy to get going at 1,319 nm based on my experience
with this SFPI at 633 nm and my home-built high resolution SFPI with a slightly
longer cavity length. However, alignment turned out to be a pain in the
you-know-what. This was partially due to my desire to do the initial test
before I had a proper adjustable mount for the SFPI (or the laser) and the
cavity spacing of the SFPI was almost certainly not optimal for the new
mirror set, but also for other reasons that will become obvious.

The laser I used for testing was the LWE-120-02 (about 4 mW) with a
single frequency output at 1,319 nm (or at least that what the specs say).
I started with a Newport IR detector intended for a power meter like the 835.
I know this to be reliable but with a mediocre frequency response. I could
always reduce the scan rate to check for display quality. And once
any display was achieved, I would switch to one of my cut-off germanium
transistors which have a nice small detector area, low capacitance, and thus
decent frequency response. But it took quite awhile
to obtain anything resembling an SFPI trace. I was even at the point of
suspecting the installation of the mirrors and checked both to be sure they
were in the correct way around and not cocked in their mounts. They were
fine. Occasionally, there would be some very low level blips that correlated
with the SFPI scan but it was very difficult to maintain these. Finally with
enough duct tape and modeling clay (just kidding), a trace appeared that
was stable for long enough to be able to approximately peak the SFPI cavity
length. Even with the Newport detector, it became obvious that this was
no ordinary mirror set. And with a stable setup, switching to the better
cutoff transistor detector was a snap.

Not only does this thing work well at 1,319 nm, but the finesse of at least
500 and possibly approaching 1,000 even without worrying about the laser's
beam size or divergence. My conclusion is that these are probably not a
standard mirror set. And recall that the finesse of
a confocal cavity SFPI is cut in half, so the mirrors must be even better.
With the scope set so that one FSR spans the complete screen, a peak is
essentially a thin vertical line, much narrower than what's typical with
the off-the-shelf mirrors or found in the brochure (above).
Only when expanded using the SPFI controller or 10X sweep on the scope
can the width be resolved. To achieve an effective (confocal) finesse of 500
requires mirrors with a reflectance of 99.7%. Since the finesse here
could be 1,000 (or more), the reflectance may
be much higher. Now I know why it was more of a pain to get going at 1,319 nm
then at 633 nm with my 98.5% mirrors. These are closer to HRs. And
of course working with a low power laser at 1,319 nm didn't help.

Not surprisingly, the LWE-120 must not like back-reflections.
There is a second mode that comes and goes but is more likely
with the SFPI peaked for maximum resolution. Misalign the SFPI
sufficiently and the second mode goes away
entirely. On the display it appears ~50 MHz away, but
who knows where it actually is due to the aliasing of the SFPI's FSR.
Because of the way everything is set up now, I can't really move the laser any
distance away - it's nose-to-nose with the SFPI - so back-reflections are
inevitable. And I don't have an optical isolator for 1,319 nm.

The standard mount that is supposed to also be used with this SFPI (that
looks more like a HeNe laser head cylinder) seems to be far from ideal.
And although I have one for the other Coherent SFPI head, I don't have
the required adapter ring as the barrel diameter is smaller than
for the short SFPIs. So, I may build a platform mount with three
adjustable feet for it.

TecOptics is currently a manufacturer
of custom optics including (fixed) Fabry-Perot etalons. However, they used
to offer a variety of standard products including the FPI-25 general purpose
plane mirror SFPI with adjustable FSR, as well as confocal cavity SFPI systems
similar to those from Burleigh, Coherent, Spectra-Physics, and others.

The SA series of confocal SFPIs consists of an SFPI body which includes a
fixed lens at the front and fine-thread adjustment ring at the back into
which the Si or Ge photodiode module is installed. The adjustment ring is
always accessible to enable the mirror spacing to be fine tuned, and may
be locked in place so the setting shouldn't change. The mirror sets are
mounted on plates mounted via two screws to the PZT assembly inside.

The HV connection is a miniature coax and the photodiode uses
a 3-pin DIN so they can't be accidentally swapped, at least not
at the head-end!

Here are some photos of an SA-7.5 which has an FSR of 7.5 GHz (mirror spacing
of about 10 mm):

TecOptics SA-7.5 Scanning Fabry-Perot Interferomter
Head - Exploded View. From left to right: Input lens, front mirror
in mount attached to PZT stack (4 sections), threaded spacer tube, back
mirror in mount on threaded cavity spacing adjuster, photodiode assembly. Of
all the commercial SFPI heads I've seen, this is perhaps the best designed
in terms of ease of adjustment and disassembly.

TecOptics SA-7.5 Scanning Fabry-Perot Interferomter
Head - Closeup of Front Mirror and PZT Stack. The coating over the 4
layer PZT stack is actually rather stiff - I had assumed it would be more like
soft rubber. But I guess at displacements of a 1 or 2 um, that doesn't
really matter. Note the small diameter of the ~10 mm RoC mirror - only
about 4 mm. The metal spacer/trip ring can be removed to reduce the
mirror spacing for use with mirrors having a smaller
RoC. For example, an SFPI with a 30 GHz FSR would require an RoC and
mirror spacing of about 2.5 mm. But then the mirror mounting screws
would have to be changed so as not to protrude (much) beyond the mirror
mount plate.

The mirror set in the unit I was given has a wavelength range of 850 to 920 nm,
which doesn't overlap any of the lasers I really care about. It might be good
for use with a Ti:Sapphire laser, which regrettably, I don't happen to have
available at the moment. :) It could also be used with single spatial mode
diode lasers. But as yet, I have not actually tested this unit, though I see
no reason why it shouldn't behave like all the others. I have thought about
making suitable mounting plates for a pair of mirrors with the same
reflectivity as those in my $2 SFPI, approximately 99 percent at 532 nm,
custom coated for a defunct project on substrates that are Melles Griot
plano-concave lenses. But since I already have an SP-470-02 (8 GHz,
as well as my $2 SFPI), this wouldn't really provide anything fundamentally
new. So, I'll simply have to find a suitable laser!

The SA200 and SA210 series of SFPI heads from
Thorlabs comes in several wavelength
ranges: 350 to 535 nm, 535 to 820 nm, 820 to 1,275 nm, 1,275 to 2,000 nm,
and 1,800 to 2,500 nm. (The mirror sets are not intended to be
swapped by the user, if at all.) The SA200 has an FSR of 1.5 GHz while the
tThe SA210 has an FSR of 10 GHz. Google will come up
with the relevant Thorlabs info by searching for "Thorlabs Scanning
Fabry-Perot" or by searching on their Web site.

The SA200 and SA210 are of a modern design with a PZT requiring only around
20 V rather than several hundred V to cover approximately 2 FSRs. (This
is similar to my PZT beeper-based home-built SFPIs!) So, while
Thorlabs does offer the SA201 control box, a normal function generator will
suffice to drive the PZT. (The SA201 also include a PD preamp.)
CAUTION: The rated input of the PZT is only 150 V, so take care if using a
high voltage ramp driver like the SP-476; add a voltage divider or
zener or neon lamp clamp to limit the maximum voltage!
Another nice feature is iris diaphragms at both the input and output
(photodetector) ends of the head. Reducing the apertures aids in alignment and
enables the effective resolution to be maximized. A small aperture also
tends to reduce the likelihood of back-reflections which may destabilize
the source laser. Neither head has an input focusing lens built-in like
the SP-470s, so an external lens with its focus approximately at the
center of the SFPI cavity may need to be added for best performance.

The SA201 Spectrum Analyzer Controller includes a ramp generator and
photodiode preamp. The ramp driver has a 10 turn pot with a nice large knob
for sweep offset, a 7 position switch for 1x, 2x, 5x, 10x, 20x, 50x, or 100x
sweep expansion (which actually selects sweep time), a button to select
sawtooth or triangle waveform, and a recessed trimpot for sweep amplitude.
There is no provision for true sweep expansion, which would maintain the
same total sweep time (or equivalently, sweep frequency), but with its
amplitude reduced by factors of 1, 2, 5, 10, 20, 50, or 100 around
the center of the sweep. Having both sweep expansion and sweep time (as with
the SP-476) would be desirable, though hardly essential. The sawtooth has a
fixed limited slew rate for its fall (retace) so there is no
risk of harming the PZT. The photodiode preamp may be set to a gain of 10k,
100k, or 1M V/A, and there is a recessed trimpot to adjust
risetime. The PZT output and scope trigger connectors are on the front
panel while the PD input and output connectors are on the rear panel.
It has a nice basic set of features and would work well with my home-built
SFPIs. :) The design is based on a Lattic CPLD for the waveform generation
with virtually no analog timing components, so it should be quite stable
over time. My only complaint is with respect to size and weight: With modern
components and a wall adapter for power, the entire controller could easily
be 1/10th as large and weigh 1/10th as much. And the power button
apparently controls a relay producing a resounding CLUNK, as though
it's using 10 kW of AC power! :)

The complete operation manuals for the SA200, SA210, and SA201, may be
found on the Thorlabs Web site.

I wanted an SFPI with the smallest FSR available (from Thorlabs) to check
the line-width of a stabilized line-narrowed diode laser at 685 nm. My
SP-470-03 is only spec'd from 550 to 650 nm so there was some question as
to whether what I was seeing there was accurate. My home-built high resolution
SFPI has mirrors that would have been suitable, but the FSR is too small.

The head I tested was an SA200-5B which has the 535 to 820 nm mirror set.
The 1.5 GHz FSR is a bit small for unambiguously monitoring the modes of red
(633 nm) HeNe lasers as outlying modes may alias on the display -
2 to 2.5 GHz would be preferred. But for most modern :) applications
like checking for single longitudinal (single frequency) performance of
diode and DPSS lasers, this isn't a problem. Although only spec'd down
to 535 nm, not surprisingly, the performance at 532 nm is still exceptional.
In fact, without careful measurements, it would be impossible to tell the
difference between the resolvance at the three wavelengths I used for
testing: 532 nm (DPSS), 633 nm (HeNe), or 685 nm (diode).
I suppose that spec'ing 535 nm may
necessary to take into account worst case variations in mirror coating
wavelength coverage. However, given the prevalence of 532 nm, it would have
been nice to be included within the spec'd wavelength range.

As it turned out, again not unexpectedly, SP's specs are also conservative
so the SP-470 display was very similar to that of the SA200-5B at 685 nm
and 532 nm, though the SA200-5B may have had slightly better
resolvance. Both these are well outside the 550 to 650 nm
range of the SP-470, but 532 nm is very close to the low-end
spec of the SA200-5B and 685 nm is near the middle of its
spec'd range.

The lack of a focusing lens also didn't seem to have any detrimental
effect on performance with any of these lasers, but they all have a
small beam size. The apertures could always be reduced (laser power
permitting) for lasers with a larger beam diameter.

The PD preamp has a wide dynamic range so it's easy to find a combination
of gain setting and scope sensitivity that fills the screen without clipping,
distortion, or excessive noise. The lack of a sweep voltage knob was not
really of any consequence. (There is the recessed trimpot.) It came from
the factory set at a full span of approximately 2 FSRs, which would be my
default setting anyhow. The sweep expansion switch provides more zoom
than could be ever be needed.

And the Lab Snacks Thorlabs included with the loaner unit were definitely
yummy. ;-)

Of course, even a perfect product can be improved, so here are some
suggestions (Thorlabs, are you listening?):

Change the FSR to 2 GHz instead of 1.5 GHz for the SA200. This is better
for use with red (633 nm) HeNe lasers where the gain bandwidth is 1.5 or
1.6 GHz. Then there is less ambiguity due to aliasing with outlying modes.

Extend the wavelength spec for the SA200-5B down to 532 nm. This is
likely only a documentation change as it's almost certain any randomly
selected 5B mirror will work very well at 532 nm.

Include a (removable) lens on front optimized for parallel beams. This
should help resolution with large beams. Stopping down (particularly the
PD iris) helps a lot, but reduces the signal and thus increases noise.

Change sweep expansion on SA201 so it references to center of scan
instead of the start and maintains the same total sweep period/frequency.
Then, if a mode is centered, it will expand without shifting position
and the sweep won't slow down proportionally resulting in fewer scans
per second on the scope scope and excessive flicker. This would also
improve the response to real-time variations in mode position, amplitude,
and width . Perhaps both options are needed so that the entire 2 FSRs can
also be scanned at slow speed, thus independent controls for sweep rate
(pot) and expansion (selector switch).

Add true sweep magnifier, which would permit a selected portion of
the display to be expanded, similar to the function found on classic
Tekronix oscilloscopes. The overall sweep rate and amplitude would
not change but a selected portion could be mangified and moved on
the display. This would be cool. :)

Add variable gain knob on PD preamp. No biggie. just a bit of a
convenience.

Build a high resolution LCD display into the control box resulting in a
fully self-contained SFPI. ;-) Compared to the price of the SFPI head, this
would represent a relatively modest increase manufacturing cost. For more
on this possibility, see the section: Sam's
Proposed Scanning Fabry-Perot Interferometer Controller.

And absolutely the most important: Include a larger assortment of lab
snacks. ;-) Also Thorlabs hat, T-shirt, cool LED pen and other cool optics
gizmos. :)

This thing showed up on eBay with a listing title of "Burleigh Laser
Exciter with Prisms (UNTESTED)". It had what appeared to be cube-corners
at both ends and a bunch of wires coming out of it. Even though the seller
provided many photos, none was very good at revealing what the device
really was. If it was indeed a laser exciter (I have no idea where
they got that name), then perhaps there was a fancy laser inside!
But the 6 skinny wires hanging out of it terminated with pin-plugs
didn't seem right for a laser and seemed more appropriate for
a bomb. :) It was listed several times over a couple
weeks with no takers and with the "Buy-It-Now" price getting lower and lower
and lower until....It finally became cheap enough that I HAD to satisfy
my curiosity!

Well, the "Burleigh Thing" as I've been calling it
turned out to be a Fabry-Perot interferometer with a PieZo Transducer
(PZT) for fine adjustment of mirror spacing. The mirrors are planar
and their separation is adjustable by changing the
position of the PZT assembly, so it can be set for a wide range of FSRs.
(Free Spectral Range - the extent over which optical frequencies are unique.)
This device could certainly serve as a Scanning Fabry-Perot Interferometer
(SFPI) for displaying laser modes, though it's more likely to have
been used as a tunable etalon or narrow-band optical filter. The
normal beam path goes through the mirrors three times, which is like
putting three SFPIs, etalons, or filters in series. One thing is for sure:
This must have cost someone (probably the U.S. Taxpayer!) a fortune. :)

Here are a few photos.

Burleigh Triple-Pass SFPI - Overall View.
This thing is massive weighing in at over 20 pounds! It sits on three
screws for overall alignment with the incoming beam. Someone must have
made a diligent effort to abuse the poor thing as I had to straighten
those, as well as the adjustment screws for the front mirror! Such damage
doesn't happen by accident on a precision device.

Reflect from CCA #1, forward through the FP, and out via the hole
in CCA #2.

The prisms were mounted using RTV Silicone with no protection, not even a
cover. So, I'm guessing this device was originally inside a larger device
out of harm's way. The CCAs are easily removable if
desired to reduce the number of passes to 2 (up and back) or 1 (straight
through). At the very least, this probably makes initial setup a lot easier!
It's also possible that they were not standard equipment but were added
for some specific application.

Burleigh Triple-Pass SFPI - Fixed Mirror
Assembly. The three large screws attaches it
to the SFPI housing. It bottoms against 3 other screws with large shoulders,
2 of which are visible next to the large ones at the bottom of the photo.
Together, these provide for overall mirror alignment. The front cube-corner
assembly simply slides into the big hole. That's the lens of my camera
reflected in the mirror.

Burleigh Triple-Pass SFPI - PZT Assembly with Moving
Mirror. There are three PZT plates about 2 inches long and about 1 inch
wide mounted at the 120 degree positions. They extend nearly
the length of the cylinder and are curved to fit. The PZT plates push on
the mirror mount, which is attached with flexible metal straps.
So a high voltage driver with three independently adjustable outputs
might be needed to equalize the deflection, and fine tune and maintain optimal
mirror alignment throughout the scan. But putting all three PZTs in
parallel would probably be good enough for government work. :) The
entire PZT/mirror assembly slides into the body of the SFPI and can
be set and locked in position over a wide range of FSRs. The distance
between the mirrors can be from almost touching (FSR of 100 GHz or more)
to about 2.5 cm (FSR of ~6 GHz) with the rear cube-corner assembly
installed in its normal position, or at least 10 cm (FSR of ~1.5 GHz) with
it removed entirely.

The design wavelength of the mirrors is not known, but they are about
75%@633nm and 90%@532 nm, which would be rather mediocre if used with
a single pass interferometer. They appear greenish in reflection and
weak pink/purple in transmission. Based on appearance and that the
reflectance is decent at 532 nm, I would suspect they may have been intended
for 1,5XX nm (roughly 3x532 nm) and have a higher reflectance there.
However, the transmission functions through the interferometers multiply when
they are used in series. So with three passes, this thing may still be
quite impressive at these wavelengths, even if they aren't optimal.
If one pass suppresses out-of-band wavelengths by a factor of 10,
with three passes, it would be a factor of 1,000.

(From: John Barry.)

"I found that the best way to consider this problem involved thinking
about the transmission function for a single Fabry Perot, given below from
the Wikipedia entry for "Fabry-Perot Interferometer". (Note: I derived
these myself just to check that it is applicable.)

For Three F-P interferometers in series, the transmissions should multiply (as
you said) giving the final transmission to be Te3 where Te is
given above. Unfortunately this curve of Te3 has a different
functional form than Te so the Finesse is not really well defined."

While I doubt that anyone will ever see another one of these beasts, for
reference, here is the wiring info for the PZTs. The polarity is arbitrary
- I don't know whether a positive voltage expands or shrinks the PZT bars:

The operation manual for a Burleigh RC-44 Ramp Generator lists color codes
that agree with these. The "+" is assigned to the HV Ramp, and the "-" to
the HV Bias (electrical adjustment for fine tuning of mirror alignment, which
I intend to add eventually). So, this color code must be used for
other Burleigh SPFIs.

With the cube-corner assemblies removed (single pass), and set for an FSR
of about 6 GHz, I aligned the
mirrors using a red HeNe laser and attached the PZTs (in parallel) to
my SP-476 controller. And the Burleigh Thing does work, sort of.
The finesse for a single pass at 633 nm is truly mediocre, but it is
able to resolve the longitudinal modes of a Melles Griot 05-LHR-911
HeNe laser (883 MHz mode spacing) - barely, as overlapping lumps rather than
peaks. The finesse is probably optimistically around 8. This poor
performance was expected given the low reflectance of the mirrors at 633 nm.
With a single frequency 532 nm DPSS laser (Coherent C215M), the display is
much better with distint peaks and a finesse of at least 15. :) Theory
predicts a finesse of about 30, but the discrepency could be due to
the fact that the beam of the C215M laser is slightly diverging. I'm
not sure I have the determination to get it working triple-pass but it
may come to that! However, driving all three PZTs using the single HV
output of the SP-476 works well enough with no evidence of a change in
amplitude over several FSRs. The PZT/Mirror assembly does make quite
a racket though - a loud ticking sound - with the SP-476's sawtooth
drive, much more so than the small SP "normal" SFPI heads. Or the
near silence of my home-built SPFIs! But being able to control the
drive voltages (or at least the bias voltages) independently would provide
fine tuning of mirror alignment, which is difficult with the large clunky
adjustment screws. More on this in the next section.

And no, I didn't attempt to get the Burleigh Thing to work in triple-pass mode
and probably never will!

I had a really fuzzy vague recollection of being shown an SFPI similar or
identical to the Burleigh Thing (but without the CCAs) a few years ago at a
local college, gathering dust in the corner of an undergrad physics teaching
lab. I mentioned to the instructor who pointed it out to me that the mirrors
didn't look like they were good for red, but I didn't realize that they
also weren't very good for green, and he probably had no clue either.
I might have also commented on how much larger and more massive it was than
the SFPIs I build. Then again, perhaps all this was just my imagination.
Or, as someone else put it: "When I was a child, I had a fleeting glimpse
of something out of the corner of my eye. When I turned to look, it
was gone and I cannot put my finger on it now." :) Now totally coincidentally,
I had the opportunity to visit the teaching lab because the SFPI was a bit
sick. And wouldn't you know, it really didn't look anything like the
Burleigh thing! See the section: The Tropel
350 Scanning Fabry-Perot Interferometer.

The original mirrors in this thing result in mediocre performance for any
lasers I care about (and that's being generous!). They were most likely
designed for 1.5 µm and produce a display barely recognizable as
a spectrum at 532 nm or 633 nm. So, I decided to attempt to replace
them with mirrors that would be useful for visible lasers. Now I just
happened to acquire a pair of nice planar mirrors. While also likely
designed for 1.5XX µm, they were probably intended to be HRs there
and the reflectivity at 532 nm is around 99 percent. As we now know based
on my $2 SFPI, an HR at 1.5xx µm will have a lower reflectance
at one third of 1.5 µm or approximately
the 500 to 533 nm range. I figured that 99 percent at 532 nm (and probably
slightly less at 543.5 nm but close enough for Government work!) would be
ideal, but that was based on a confocal cavity, not plane-plane for the
Burleigh thing. I did not realize how difficult alignment would be with
the planar mirrors and not so fantastic Burleigh mirror adjusters.

Removing the original mirrors wasn't going to easy because they were glued
into recessed tight fitting holders. And in fact, for various reasons,
they did not survive very well. So, there was no going back. I had
to make the replacement mirrors work.

The original mirrors were about 1.5 inches in diameter and the replacements
were 2 inches in diameter. My plan was to glue them to the lips of the
original holders with 5 minute Epoxy. Hopefully, the resilience of
the Epoxy would minimize any stresses on the mirrors that might distort them.
And these mirrors were about half the thickness of the originals.
This approach worked nicely for the back mirror on the PZT
because that had plenty of clearance. But what I had not anticipated
was that the front mirror fit through a hole in the end-plate attached
to the SFPI body and that hole was only slightly larger than the mirror
holder, and was definitely less then 2 inches! So, the plate had to come
off. But then the larger diameter mirror would prevent the mirror mount
from ever being removed without ungluing the mirror. That might be an
annoyance in the future but of more immediate concern was the fact that
the mirror mount blocked access to the screws securing the end-plate to
the SFPI body when it was installed. It seemed like a catch-22 situation
that would require some tiny robots to go inside and install
the mirror after the end-plate and mount were screwed back together.
But as it turned out, there was just enough leeway to permit a ball-end
hex wrench to get in between them to tighten the screws to reattach the
end-plate even with the mirror attached.

So, both replacement planar mirrors were installed without incident. That
was the easy part.

Mirror alignment is always a royal pain with a plane-plane Fabry-Perot
(F-P) cavity and even more critical with a high finesse F-P cavity. The
mirror adjusters on this thing are to put it mildly, not ideal for fine
control. OK, not putting it mildly, they stink. :( :) The construction is
simply three screws pulling the mirror mount plate toward the
end-plate attached to the SFPI body, and another three screws behind the
mirror mount plate holding it back. No springs or even springy washers!
So, once the alignment is close, it's a matter of tightening each pair
of screws against each-other while attempting to do the fine tuning.
Optimizing alignment is a matter of observing the scatter off the mirror
from the multiple reflections and forcing them into as tight a splotch
as possible with these clunky adjusters, which also tend not to be totally
independent of each-other. Admittedly, this was designed to be aligned
once and locked in place forever. Then likely fine-tuned by adjusting
the bias voltages to the 3 PZTs. (More on this below.)
So, fancy mirror adjusters wouldn't
be justified. And 1,000 tpi screws or a differential screw scheme
would be required to get the sort of resolution needed to really
make manual alignment less of a royal pain.

As if this weren't enough, in a plane-plane SFPI, the laser must be set up so
that its beam is perpendicular to the input mirror so that its beam is
reflected precisely back on itself. Now, most lasers get mighty unhappy when
this is done and make their hurt feelings known by becoming unstable and mode
hopping all over the place, even off the optical table. :) But if the beam
is at an angle to the input mirror, not only is the finesse achievable
much lower, but there is no way to judge when the
mirrors are parallel to each-other, and when they aren't, the
scatter pattern becomes a curved rather than a straight series
of spots, making it much more difficult to even figure out which
way to turn the alignment screws.

So, the laser must be aligned to the input mirror hoping it doesn't
complain too much. An optical isolator can be quite effective but is
also quite expensive (BIG $$$), although a polarizing beam-splitter and
Quarter-Wave Plate (QWP) might work if the laser is polarized. An optical
filter can also be used to reduce the intensity of the return beam at the
expense of usable output power. At first, I was using a 5 mW green
laser pointer that had a real on-off switch and was designed for more or less
continuous operation. But the transmission through two ~99 percent
mirrors was so low that only in a totally dark room, was it possible
to begin to see anything coming through. Even so, that did enable
the SFPI to be more or less aligned and to produce a viewable signal.
But the pointer was probably multi-mode on its own and really
complaining with constant mode hops, oscillation, and noise.
However, these mirrors are very narrow band and almost useless for a green
HeNe laser at 543.5 nm, which I tried next. So, I switched to a ~20 mW
Coherent C215M laser. Its higher power made life a lot easier and for
the most part, it seemed immune to the back-reflected beam, though there
was some jitter and the occasional mode hop (which may simply have been
due to the C215M case not being temperature-controlled).

The two SPFI mirrors must be aligned parallel to each other to
a very small fraction of a mR - much higher precision than even for a
large frame (narrow bore) HeNe laser. And with the mirror adjusters
locked tight, just pushing on the massive mirror plate still resulted
in a detectable change in the signal. I ended
up using rubber wedges between the mirror mount
plate and backing plate to fine tune it since it was impossible to
use the adjusters alone to peak the signal. There was too much friction
and backlash to make precise enough tweaks. A ramp generator with
individual bias settings for each PZT would have helped with
the fine alignment, but I didn't have one handy.

So, I built a little bias box for use with the SP-476
(or any other single output ramp generator). It has a built-in
200 VDC power supply (salvaged from a particle measuring system,
probably for an avalanche photodiode) so it can be used with any
ramp generator. Three pots provide independent bias to the
normally grounded ends of the PZTs. There are 100K ohm resistors in
series with the PZTs for protection against shorts in the PZTs or wiring,
and 0.1 uF capacitors between the pot wipers and ground to bypass any
feed-through of the drive voltage via the capacitance of the PZTs. (The
operation manual for the Burleigh RC-44 Ramp Generator shows the PZTs
being composed of separate ramp and bias sections with the center point
grounded. But the Burleigh thing only has single PZT elements. Thus
the bypass capacitors may be needed.) With the pots more or less centered,
the mechanical alignment can be performed so that the display looks decent
with tall narrow peaks, but it doesn't have to be the absolute best
possible. Then, the three pots provide convenient and repeatable
optimization.

Since even with near perfect alignment, there is inevitable walk-off
and spreading after a zillion reflections, a small photodiode or a small
aperture in front of the photodiode also helps to improve resolution. In the
end, I used a photodiode with an active area of about 1x1mm.

With careful setup (laser, mirror, and photodiode alignment), the Burleigh
thing is now capable of a finesse of 200 to 300 at a wavelength of 532 nm.
For some reason, when adjusted for best resolvance the display has a
skinny peak sitting on top of a wider pedestal, with the FWHM point being
well above the wide section. The pedestal is assymetric though, more spread
out on one side than the other. But this is not an electrical issue such
as a slow photodiode preamp - the spread out side swaps if the polarity
of the PZT drive is reversed. If a positive input to the PZTs decreases
cavity spacing (by increasing the length of the PZT element), the wide side
is on the slope of the peak going towards smaller mirror
spacing. It almost has the appearance of one or more additional lasing
modes just at the limit of resolvance, but this laser has been tested
with another SFPI and is known to be pure SLM. And multiple lasing
modes so close together that remain stationary with respect to the
main mode as these do would be extremely unlikely or impossible anyhow.
So, this is very likely simply a result of imperfect alignment as the
peak can be increased to a decent height relative to the pedestal
with enough fiddling. :) The higher ratio of peak to pedestal also translates
into better finesses. One of the better results so far
(with the replacement mirrors) is shown
in Burleigh Thing Plane-Plane SFPI Display of 532 nm
SLM DPSS Laser. The unequal heights of the two peaks is likely due to
differences in sensitivity of the three PZTs, which has not been corrected
since they are driven in parallel. But this does show how sensitive the
finesse (and thus peak height) of a plane-plane F-P cavity is to mirror
parallelism: A miniscule change in alignment over a movement of less
the 1 µm (with a mirror spacing of over 37 mm) is enough to
produce a sizable effect. The relative heights of multiple peaks
in the display can also be changed at will with the mirror bias pots.

The main benefit of a plane-plane SFPI is that the FSR can be varied
over a wide range by changing the distance between the mirrors. But
even with the non-adjustable mirror (and PZT) mounted in cylinder that's
a precise fit to the SFPI body, moving it any significant distance will
mess up alignment, requiring going through mechanical alignment all
over again!

So the conclusions of this exercise (and my plane mirror SFPI)
may be that attempting to make a high finesse plane-plane SFPI
is not worth the effort unless it's absolutely needed, usually to
achieve a higher FSR than what's practical in a confocal SFPI due
to the very small highly curved concave mirrors that are required. The
highest FSR for a commercial confocal SFPI I know of is 30 GHz from
Coherent. Those mirrors, which have an RoC of only 2.5 mm, look
like the interior of a pea that's been cut in half with each being
a good portion of a full hemisphere. How are high quality
mirrors like that even ground and polished? And confocal SFPIs
from most other manufacturers only go to 8 or 10 GHz.
A plane-plane SFPI can go up to over 1 THz! But if this is required, start
with a precision instrument like the TecOptics FPI-25 or Tropel 350
described below. As a practical matter, though, a monochromator-based
Optical Spectrum Analyzer (OSA), while pricey, is probably a
better and easier instrument to use with high bandwidth lasers.

This small head has no model number so it might have been custom and
intended as a tunable etalon though it was in the standard Burleigh
4-axis gimbal mount as shown in
Burleigh Plane-Plane Scanning
Fabry-Perot Interferomter Head in Gimbal Mount.
At first I assumed it was a normal confocal SFPI head
with IR (1,5xx nm) mirrors (based on appearance) but the behavior was strange:
It was not possible to even obtain "lumps" for a display using a green (532
nm) DPSS laser (which should have worked at least marginally at 1/3rd the
design wavelength). The reason became obvious once I realized that the mirrors
were planar - my ramp generator was driving only one of the three PZT stacks
so the mirror was tilting rather than translating. Several view of the head
alone are shown in Burleigh Plane-Plane
Scanning Fabry-Perot Interferomter Head. The bottom views are looking
into the head with the rear focusing lens and photodetector assembly
removed.

Testing with a Melles Griot 05-LIR-151 1,523 nm HeNe laser and ramp driver
wired to all three PZTs in parallel, resulted in a good enough display to
deduce the FSR. The distance between longitudinal
modes of the 05-LIR-151 is 438 MHz appearing at 1/32nd of the distance
between where the display repeats for an FSR of around 14 GHz. In the
interest of round numbers, I'm guessing it is actually 15 GHz. :) It may
that the mirror spacing is adjustable but I have no idea how.

Three screws at the back of the head provide for coarse mirror parallelism
adjustment with the PZT bias settings doing the fine alignment. Then the
Pan and tilt of the mount are used to set the beam to be exactly normal
to the mirrors. This, of course, results in a strong back-reflection directly
into the laser, so ideally some type of optical isolator is required.
However, simply attenuating the beam with a neutral density filter allowed
for acceptable stability. The finesse is poor - optimistically 75 - but
this may be due to imperfect mirror parallelism, the slightly expanding
beam of the laser, and the coatings not being optimal for 1,523 nm.

The Burleigh RC-42 is a high voltage ramp generator includes
the usual ramp amplitude and speed controls as well as three bias
controls and three gain trim-pots. These provide for electrical fine
tuning of mirror alignment and compensation for unequal PZT sensitivity on
interferometers with triple PZT stacks like the Burleigh triple-pass
SFPI described above. It can also be used with single-PZT instruments
by ignoring 2 of the 3 outputs.

Here are some relevant specifications, determined by testing. All values
are approximate:

Burleigh Model RC-42 Ramp Generator - Back shows
the array of BNC connectors for sync and scan monitoring, and the DB9F
for the high voltage PZT outputs. Even if the maximum PZT output is only
a few hundred volts, a DB9F seems a bit wimpy. But the benefit is that
it is a standard easy to find inexpensive connector as opposed to the
fancy round one Burleigh switched to in later models.

The RC-42 is both larger and heavier than it appears in the photos. At first
I was wondering if there might be vacuum tubes inside! The power transformer
may be from the tube era - a Stancor 8418 (230-0-230 V rated 50 mADC, 6.3 V
at 2.5 A) but it is all solid state and even has several ICs, though a few
of them are TO5 cans jammed into 8 pin DIP sockets for some reason! As it
turns out, much of the weight is in the thick steel chassis and cover. That
doesn't help it to look any smaller though. :-)

The TecOptics FPI-25 is a general purpose instrument with a plane-plane
cavity that can be set for an FSR of more than 300 GHz (mirrors almost
touching) to a much lower FSR when the mirrors are at their maximum
extent. They accommodate easily interchangeable
mirror sets covering several wavelength
ranges. There's no reason the FPI-25 couldn't also be
configured as a confocal SFPI with suitable mirrors.
Of course, like other voltage-controlled
Fabry-Perot etalons, it may also be used
in other ways in addition to as an SFPI for CW lasers. With a slow
ramp, it can generate the time-averaged spectrum of a quasi-CW laser,
and with a fixed (or feedback-controlled) drive voltage, it can
act as a tunable optical filter.

The FPI-25 is similar in basic capabilities to the "Burleigh Thing"
described above (when used with only a single pass) but has a somewhat
better set of adjustments designed to be used for optics experiments.
Cavity spacing is varied by a knob turning a fine-threaded rod that
moves the massive Invar resonator assembly. It may be locked in
place once the desired position is
reached. Three knobs which turn differential screws are used for
extremely precise mirror adjustment, though this is considered
"coarse" compared to what the PZTs can provide. The triple PZT
with its mirror is fixed to the base. Changing cavity spacing still may
require fine tuning of the mirror alignment, but not by much so that
individual control of the three PZT offset voltages may be enough for
this. The entire instrument sits on a pan-tilt mount so aligning it
to the input source is quite easy. It may also be removed from the
base and mounted on a (sturdy) post.

Based on operation manuals I've seen, some versions of the FPI-25
were sold for awhile by Melles Griot, possibly after TecOptics gave
up or sold the interferometer product line. The manual for
systems that appear identical to those from TecOptics
lists three models differing only in the maximum cavity
spacing/minimum FSR: W1000 (60 mm/2.5 GHz), W2000 (100 mm/1.5 GHz),
and W3000 (150 mm/1 GHz). There were at least two
ramp generators. The basic version (FPZ-1-RG, photo
below but not referenced by Melles Griot) doesn't have individual
controls for bias and offset as does the fancier FPZ-3-RG.
Melles Griot also offered an updated version of an optical head similar
(but not identical) to the W1000 (50 mm/3 GHz) with their own redesigned
ramp generator - the 13-FPC-001. (At least it has a more stylish front
panel!) The main change to the optical head aside from reducing the FSR
slightly seems to be that the resonator with the adjustable mirror
and photodiode assembly is attached to the base and the fixed mirror
on the PZTs is what moves to vary the FSR. However, as of 2010, there are no
references to either system or their components on the Melles Griot Web site.
But here are the two manuals (with permission from Melles Griot):

Both manuals also include nice sections on basic interferometer theory.

I have used an original TecOptics FPI-25 with the FPZ-3-RG, but don't
actually own one. That system had a separate photodiode preamp box,
the DA-1 (mentioned in the Melles Griot manual but with
no description). Here are some photos of a TecOptics FPI-25 from
an eBay auction. This would be equivalent to the Melles Griot W3000.
I should have bought it but wasn't an SFPIs enthusiast back then. :)
(I'll be happy to acknowledge the source of the following photos if
the owner will come forward):

Note that the photodiode assembly can be mounted at either end of the
optical head but calling the end with the fixed mirror the front and
using it as the input normally makes more sense since adjusting the
interferometer mirror alingment doesn't then affect laser-to-interferometer
alignment.

TecOptics FPI-25 Scanning Fabry-Perot
Interferometer - Front View. The knobs for pan and tilt are
visible near the front. A pair are on top of the baseplate (to set
the unit horizontal) and the other on the side. The resonator spacing
adjustment knob turns a threaded rod that moves the entire
resonator assembly (with adjustable mirror) relative to the
fixed mirror on the PZTs attached to the base.

Tropel was a manufacturer of a variety of laser-related equipment including
interferometers and stabilized HeNe lasers, but apparently sold off these
products to Coherent, possibly in the late 1970s. I don't know if the
Tropel metrology division of Corning
is a descendent of the same Tropel.

The Tropel 350 is a real beauty, but a monster. OK, a beautiful monster!
Or, would be if it was cleaned of years of neglect and given a nice polish. :)
It has a heavy 4-bar (probably Invar) frame
roughly 8x8x12 inches overall, and weighs in at over 30 pounds.
Everything is visible which make it extremely useful in a
teaching lab (which is where this one is located).
It has a large planar mirror mounted on one end-plate and a large
planar mirror that can be positioned along the
length of the resonator by loosening some set-screws and moving it on the rods.
The fixed mirror has three very smooth precise micrometer adjustments. The
plate on which the movable mirror is mounted is attached to it
by three cylindrical PZTs (about 1-1/4 inches long by 3/4 inch in diameter)
that are fully exposed. The mirrors are 2 inches in diameter.

The controller is fairly basic with 10 turn pots for scan amplitude and
offset, a selector for speed, and three bias adjustments for fine mirror
alignment. A Thorlabs photodetector with focusing lens (of course not
original equipment!) was mounted externally. (Though there really is
no "inside" to this SFPI!)

Although there was a Burleigh manual with the setup, this appears to be
Tropel through and through (unless the two companies had some connection
in the past). There were no photos or diagrams of the equipment in
the manual, so it was rather generic.

The design wavelength of the installed mirrors is not known but it is
definitely not for red and not for green. The reflectivity for 633 nm
(red HeNe) is probably less than 75 percent and the reflectivity for
543.5 nm (green HeNe) is probably around 90 percent. (It might be slightly
better at 532 nm.) Using a JDS Uniphase 1674 green HeNe laser results in
really dreadful finesse - optimistically maybe 20. With the mirror spacing
set for an FSR of 3 GHz (about 2 inches), it can barely resolve the
longitudinal modes spaced 325 MHz apart. The display looks more like
hands with upward pointing fingers than nice narrow peaks. :)

So, the mirrors might be designed for argon ion blue wavelengths but there
was no suitable laser to try. Or, as with my Burleigh Thing, they may be
designed for 1.5 µm. However, this instrument probably predates
the telecom age (and any need for 1.5 µm) by a few decades!

I've offered to find a set of 98 to 99 percent 633 nm mirrors that could be
installed (possibly with an adapter). These would result in a much more
respectible finesse - 150 to 300 under optimal conditions, but at least
half of this with ease. Anything over 1/2 inch in diameter is probably
adequate, though 1 inch would be desirable simply to prevent the monster from
looking totally silly with tiny mirrors. :-)

Here are some photos (coming soon):

Tropel Model 350 Scanning Fabry-Perot Interferometer -
Front. I'm calling this end the front only because that's how it was
being used. In fact, it might be better for the other end to be used for
input because the mirror alignment with respect to the source (laser) is
fixed (except for the very small effect from the bias adjustment pots).

Tropel Model 350 Scanning Fabry-Perot Interferometer -
Mirrors. These large mirrors may have been intended for a wavelength
around 1.5 µm. So there is some reflectivity at a wavelength of
543.5 nm (green HeNe, roughly 1/3rd of 1.5 µm), but the resulting
finesse is only around 20. It might be better at 532 nm (green DPSS)
or even something shorter.

I even found a reference to the use of the Tropel 350 as a triple-pass
SFPI like the one described in the section:
The Burleigh Triple-Pass Scanning Fabry-Perot
Interferometer, with Tropel RC22 cube corners and RC70-B4 2" mirrors
(whatever those are!). Unfortunately, the authors don't mention why this
configuration was used. (And the rest of the paper is absolutely boring.)
Google easily found several other papers referencing the Tropel 350 so it
must have been fairly popular (as these things go) at some point in the
past.

The reason I was able to play with the Tropel 350 was that I received an
urgent email from the lab instructor (who I had sold a one-Brewster laser to a
few years ago) that the SFPI seemed to be misbehaving. It was occasionally
making an electrical arcing sound and then ceasing to move the mirrors.
You probably never realized that laser doctors make house calls! :-)
In the end, all I could find was that perhaps there was some dirt or
some other contamination causing an intermittent short circuit in one
of voltages to the PZTs. It sounded like it was originating at the PZT itself,
but it could have been in the HV connector or elsewhere since the resulting
rapid change in voltage would make the PZT and mirror act like a loudspeaker
of sorts. And unplugging and replugging
the HV connector and jiggling wires seemed to make it go away, at least
temporarily. So, I recommended carefully cleaning the exterior with alcohol
including the HV connector and to email me in the morning. :) This would also
greatly approve the appearance for the photos I requested! If external
cleaning didn't help, then it would be necessary
to disassemble the movable mirror mount to get inside the PZTs for
cleaning, but that too should be very straightforward.

Later I found a question about this same instrument with the same arcing
problem 7 years ago on a teaching apparatus list server! Here is the
only relevant reply:

"The one that we used to use was modified for an MHV terminal for the
high voltage piezo-drive to correct arcing from the previous connector.
This also allowed us to use our own driver (that one died).

There are special spring-like washers and insulators that keep the cell
from shorting, perhaps the piezo is touching somewhere in the tube? Or
the drive lead has detached from the ceramic?

If it is like our old one, you must make a tool, like a hollow 3/16"
tube with a 1/16" "tee" pin at the end, (shaped like a T) to tighten
and adjust the cell after disassembly. This is for a lens ring that
held it in, but also lets the beam through.

Good luck, S. Anderson"

The instrument described in this posting sounds like it has
only a single PZT, but the advice about replacing the connector is
even more valid for one with three PZTs! The original 6 pin connector
is definitely not rated for 1,000 V and would be a prime suspect, especially
given that unplugging and replugging it seemed to make a difference. The
replacement doesn't really need to be an official high voltage connector.
A Molex or AMP multi-pin nylon shell would have the needed dielectric
strength, especially if only every other position were used. In addition,
I'd suggest adding a resistor in series with each PZT to protect the driver
should a short occur in the PZT assemblies. Something around 100K ohms should
provide adequate current limiting without excessively distorting the drive
waveform. (This doesn't really matter if it uses shunt regulators like
the SP-476.)

This one looks more like a fat 20 mW HeNe laser than an SFPI.
There was no model number on the one I have but based on photos and
a spec sheet, it is probably a CFT-500 with an FSR of 150 MHz and
minimum finesse of 125. However, it's also possible this was a custom
unit and may have been intended as a tunable etalon (filter)
rather than as an SFPI, though the difference is simply in application.
(But there is no built-in photodetector as would be desirable for an
SFPI.) More likely, the detector simply vanished.
The unit consists of a HEAVY beautifully
machined chrome-plated resonator cylinder about 22 inches in length and 1-3/8
inches in diameter installed in an insulated heater jacket with a 1.5K ohm
thermistor temperature sensor for thermal regulation. The front mirror is
mounted in an end-cap that has threads for fine tuning the cavity length and
a ring to lock it in place. The back mirror and PZT is in an end-cap
that screws on and seats against a fixed joint. It also has the 1K ohm
thermistor and an SMA connector for the high voltage PZT drive. Both front
and back have glass windows to prevent contamination. I don't know if it is
intended to be hermetically sealed but it does come at least close with
O-rings on all joints. And there is a small flat-head screw also with
O-ring seal to perhaps allow for the pressure to be equalized - or
something. ;-) The distance between the mirrors is just about 19.7
inches (0.5 m) for an FSR of 150 MHz. This entire assembly -
which weighs in at around 8 pounds - is housed within the insulated
heater in a light-weight aluminum cylinder about 23-1/2 inches in
length and 2.5 inches in diameter. The total weight is
about 11 pounds. There are separate cables for the high voltage PZT drive
and the heater/thermistor.

I acquired this unit on eBay. Well, parts of one plus a second
outer cylinder with insulated heater and not much else.
In fact, on eBay they were listed as "Burleigh Lasers". :) At
first I was unable to identify the wavelength of the mirror set. It had the
appearance of having high reflectance for the all too common (and useless to
me) 1,5xx nm IR wavelength - blue/green in reflectance and pale pink/orange
in transmission. However, these usually have at least some amount of
reflection at 532 nm. This had
very little - under 25 percent. But using an Ocean Optics USB2000+ with a
tungsten lamp to get an idea of the transmission in the visible range,
there was a very obvious stretch of low transmittance from around 475
to 510 nm. Indeed, at 473 nm, the transmission is around 1 percent
and at 488 nm the transmission is only around 0.2 percent. Bingo! This
should have a nice high finesse for the quite common 488 nm Coherent
Sapphire OPSL and other modern replacements for the 488 nm argon ion laser.
However, this does not appear to be one of the standard mirror sets, which
have a wider wavelength range (450 to 550 nm would be the relevant one).
The finesse may be more than 750 at 488 nm for a resolution of better
than 200 kHz. It's still possible that the design wavelength is something
other than 488 nm. It could even be an IR wavelength 3 times one somewhere
in the range of 475 to 510 nm, in which case its reflectance would be
closer to 1 and the finesse could be higher by as much as a factor of
10 or more.

Unfortunately, one of the first things I found out was that the PZT with
mirror attached had broken free of its glue and was dangling by a
single wire. (Not surprizingly, there was a scribbled note on the
outside of the outer cylinder: "Broken, please fix.". The
seller didn't mention that or show it in the auction photos!)
A chunk of the PZT material was also missing from the PZT cylinder,
pieces bouncing around elsewhere. The good news is that neither mirror
appears to have been damaged and the missing piece of the PZT probably
won't make that much difference in performance. Re-attaching the PZT
cylinder was a bit of a challenge though because it is recessed inside
the end cap of the main SFPI cylinder and that doesn't come apart.
I was afraid at first that it wouldn't be possible to assure decent
mirror alignment. But at what is probably the original orientation,
it appeared to seat square to the axis. Some careful application
first of 5 minute Epoxy in 3 equidistant spots, then slow-cure
stronger stuff in between seems to be satisfactory. The hole where
the missing bits of PZT used to be came in handy though as the electrical
connection to the outer surface of the PZT cylinder used to be via spring clips
between it and the inner surface of the metal end-cap. (I have no explanation
as to why a wire wasn't soldered to that like the inner surface.) There was
no practical way of installing them ahead of time, but once the glue had
cured, it was a simple matter to use a dental pick to carefully insert
a clip on each side of the hole. (I never did find a 3rd clip, though
I suspect there may have been 3 originally.) Part of the SMA socket
had also broken off and was stuck in the cable plug. That was soldered
back in place. It broke off again later, so a wire was simply soldered
to the connector and attached via the screw that sealed the interior.
The wires to the thermistor had been cut flush, so the
remains of the cable had to be drilled out and the wires were then reattached
directly. Aside from these minor problems, it was in perfect condition. :)
But the final result should be very close to what it was when new.
Burleigh High Resolution Scanning Fabry-Perot
Interferometer (Tunable Etalon) shows the resonator cylinder, heater
(foam insulating cylinder not shown), and outer aluminum cylinder, along
with closeups of the front and back of the resonator
and the interior of the rear end-cap housing the PZT and rear mirror.
(My designations of front and rear are of course arbitrary.)
The SMA connector is for the PZT HV, the twisted pair is for the temperature
sensor, and the flat head screw provides access to the interior. The
mirrors are pale orange/pink in transmission and blue in reflection, so
lighting and angle determined how they appear in the photos.

The rear mirror is glued into a holder which attaches to a flange glued
to the end of the PZT cylinder via the 3 screws that are visible in the
bottom middle photo. The other end of the PZT cylinder is simply glued
to the end-cap, deep inside, adjacent to the window. Repair would have
been somewhat easier if the window were removed, but that could not be done
non-destructively. So, I had to use a long narrow stick to reach
in and apply glue taking care to avoid getting it all over the
window and inside of the PZT cylinder. This was mostly successful. ;-)

My initial test with a JDSU 2201 488 nm air-cooled argon ion laser
was inconclusive as the signal was extremely noisy with the PZT drive
turned off, even after minimizing back-reflections, but maximum amplitude
when the interferometer was perfectly aligned. The noise wasn't apparent
with the photodiode directly in the beam. So, perhaps, it was actually
frequency jitter due to high ripple in the ion laser power supply
interacting with the highly selective interferometer. Yes, I know this
is grasping at laser straws.... However, the expected spots did appear at the
output indicating that alignment is probably acceptable. And the PZT makes
nice ticking sounds when driven with my SP-476.

Final testing had to await a suitable narrow line-width low noise 488 nm
laser. A Coherent Sapphire 488-50 was the perfect candidate. It was first
checked on a Spectra-Physics 470 SFPI head, which has an 8 GHz FSR, and
was found to be pure single longitudinal mode from 12 to 50 mW (the range
over which output power can be adjusted via RS232) both when stable and
during the transitions. Then the Burleigh high resolution SFPI head was
mounted on the same platform used to test the
Ultra-High Resolution Scanning Fabry Perot\
Interferometer.

The results so far are disappointing. The PZT works well, with ~200 V
to span 1 FSR. There is no evidence of asymmetry from the missing chunk
of PZT. But the maximum finesse appears to only be around 100, not the
~750 or more I was expecting. No amount of alignment was able to reduce
it significantly. I'm quite sure the mirrors are clean and aligned well
enough (parallel to each-other and centered). I did try several random
focusing lenses at the input with no improvement. Proper mode matching
at the input could still be the key though. But now I wonder
about the line-width of the Sapphire laser. Could it be that the SFPI
resolvance is actually much better than the laser and what's being displayed
is really the laser line profile? There are no published specifications
for its line-width. For that matter, Sapphire lasers are not even
guaranteed to be SLM, just that many like this one are SLM. The
Sapphire is a doubled OPSL - Optically Pumped Semiconductor Laser. But
will its line-width be closer to that of a diode or a solid state laser?
My inclination is that the wide peaks are still likely due to an SFPI
issue but cannot be certain at this point. Stay tuned for exciting
future developments.

Monochromators

While there are many ways of determining the wavelengths produced by a laser
or other light source, the simplest one beyond the use of calibrated eyeballs
is probably a monochromator. It's possible to construct one from
inexpensive parts but they also show up surplus by themselves or as
part of other optical devices like spectrophotometers, DNA analyzers,
fluorescence spectrometers, and other lab equipment with even more
obscure names.

A monochromator is an instrument which accepts a light source as its
input and can select not quite a single wavelength, but a narrow band
of wavelengths. A monochromator is probably the simplest device for
determining wavelength of a laser or other light source where standard
calibrated eyeballs aren't sufficient. (The longer harder to pronounce
term "monochronometer" is also commonly used but it refers to the same
type of device.)

There are many types of monochromators but here we only describe one of
the simplest, consisting of the following:

An Entrance (or input) slit: which determines the width of the
input light beam that will get through.

A Diffraction grating: that can be rotated to precise and
calibrated positions by a lever-lead screw affair using either a manual knob
or stepper or servo motor system. Only one of the first order diffracted
beams is selected. The diffraction grating will be blazed to maximize this
first order beam.

An Exit (or output) slit: which in conjunction with the entrance
slit determines the wavelength band.

The arrangement above using a planar diffraction grating is acceptable if
the input light source is well collimated and aligned with the
monochromator's optical axis. But, making the diffraction grating
slightly concave with its two focal points at the slits makes the system
less sensitive to the orientation or divergence of the input beam
and provides better selectivity since it is essentially imaging the light
at the entrance slit into the exit slit. Alternatively, spherical lenses
or mirrors can be used with a planar diffraction grating to achieve the
same effect.

Due to the way a diffraction grating selects wavelength, if the linear
travel of the lead screw is converted into rotation of the diffraction grating
by a lever of the proper length, the result is a linear relationship between
the "nut" location on the lead screw and wavelength. As long as the pitch
(lines/mm) of the diffraction grating is known accurately, the relationship
will be exact. Thus, a simple multiturn precision dial can be used to read
off wavelength. Or, for an automated instrument, a stepper motor will have
a constant nm/step size.

A fully optical monochromator - with no electronic detector - is perfectly
adequate and may actually be preferred for measuring laser wavelengths
in the visible range at least since almost any laser is powerful enough
to result in a beam at the output of the monochromator to be easily seen.
However, for measuring the spectrum of something like a glow discharge
(as in the bore of a HeNe laser tube) or UV or IR lasers, a high
sensitivity detector is essential. Spectra for varioue elements and
compounds can be easily found by searching the Web. The
NIST Atomic Spectra
Database has an applet which will generate a table or plot of
more spectral lines than you could ever want.

CAUTION: When exploring the interior of a monochromator, DO NOT touch the
surface of the diffraction grating. Cleaning a grating without damage is
difficult at best, and may be impossible for some types of gratings

The two mirrors allow the input and output beams to be co-linear but have
no other effect. A variety of input and exit slits permit the resolution
and sensitivity to be easily changed.
All inner surfaces of the monochromator as well as some additional
"Absorbers" are coated with a super flat black material to absorb as
much stray light as possible. This includes both unavoidable scatter
as well as the zeroth and higher order diffracted beams (not shown)
from the grating.

In the diagram, a hypothetical light source consisting of red and green lines
is shown, perhaps from a very strange Ar/Kr ion laser. The green beam is
diffracted less than the red beam and is thus not passed through the exit
slit.

Assuming the mechanical design of the monochromator is correct, only
two adjustments are needed for calibration: The angle of the diffraction
grating with respect to the lever, and the dial setting with respect to
the lead screw.

When I found the 1200VIS, both of these were far off. Simply adjusting
the dial to coincide with a 632.8 nm red HeNe laser resulted in a green
532 nm DPSS laser pointer reading 529 nm. This indicated that the lead screw
wasn't moving the lever enough over that range. To remedy this, I slightly
loosened the set screw locking the lever to the diffraction grating shaft,
but still tight enough so that normal dial twiddling wouldn't affect the
relationship. Then, using the 632.8 nm and 532 nm lasers as references,
the diffraction grating was rotated incrementally with respect to the lever
until the difference between the readings was exactly 100.8 nm. It would
have been even better to use more extreme wavelengths like a 457 nm DPSS
or argon ion laser and 647 krypton ion laser, but these will have to do
for now. Checking some other known wavelengths including 543.5, 594.1 and
611.9 nm (green, yellow, and orange HeNe lasers), as well as a 640 nm (errant
laser line being produced by the red HeNe laser) showed them to be quite
accurate.

For determining laser wavelengths, a simple white card is all that is needed
on the output. However, in its original application of detecting spectral
signatures of plasma flames and such, a sensitive photodiode or
PhotoMultiplier Tube (PMT) detector would be mounted beyond the exit
slit.

The overall performance (including wavelength precision and repeatability)
using the dial, is now better than 0.5 nm, limited by very noticeable
backlash in the multiturn dial mechanism.
Originally, the 1200VIS also had a stepper motor (which I removed),
PMT detector, and controller with data acquisition system (all whereabouts
unknown). That was probably much more accurate but for my intended uses,
this will be fine.

These devices have been turning up on eBay lately in a variety of
flavors, both manual (EP200Mmd) and motorized (EP200Msd). The latter
is really only more useful if one of the mating controllers
is also acquired - or one is built. Contructing one would not be that
difficult with a microcontroller and stepper motor driver. Both versions
include a micrometer adjustable monochromator, PhotoMultiplier Tube
(PMT) detector, its high voltage power supply, and preamp, all built into a
case about 7.5" x 7.2" x 2.6" inches. It runs on +/-15 VDC.

As its name implies, the optical output of the EP200's monochromator is sent
directly to a detector inside the unit which produces a voltage between 0 and
+10 V proportional to light intensity at the selected wavelength. There is no
exit port for light. The case is very well sealed against stray light -
the light goes in but it never comes out. :)

An annotated photo can be found in Verity EP200
Monochromator/Detector Organization. In addition to the parts being
labeled, the beam paths for the a sample input, zeroth order (reflected
input), and first order (useful) spectral lines are shown. The input here
may be from the bore discharge of a HeNe laser tube. The wavelength micrometer
is set for 587.6 nm in the photo thus selecting the intense yellow (helium)
line which passes through the exit slit to the PMT. Of all the other
lines shown (there are many more in the spectra of He and Ne), only the
green one even makes it to the vicinity of the exit slit, but it's off
to one side. Note that only the central ray for the incoming beam and
each of its spectral components has been drawn on the photo. However,
for a diffuse source like a glow discharge (as in this example), light
bulb, or even an LED, the internal beam will expand to fill the large
concave holographic diffraction grating (which provides high light gathering
power). The bounce mirrors must also be larger than what might be expected
due to the size of the internal beams. Only with a collimated laser, would
the actual beam paths closely resemble the narrow ones shown.

This model uses an actual machine shop type micrometer assembly
to select wavelength. While not as convenient as a direct reading
multidigit dial, rotation selectivity is better since there are only
25 nm per revolution compared to 100 nm for the typical dial. And,
there is no backlash in the readout so the precision is better.
However, when using narrow slits, the wobble in the micrometer
becomes significant. The motorized version with its long shaft
may be even better but it's useless without the matching controller
because there is no readout.

The sensitivity using the PMT is truly amazing. With the HV nearly as
low (close to 0 V) as it can be, -200 VDC, and the gain turned nearly all
the way down (10 percent), simply placing a small neon lamp power indicator
near the entrance slit overloaded the preamp. Numerous lines in the neon
lamp spectrum could be easily found. Since the PMT provides most of
the amplification, the preamp really isn't that sensitive in the grand
scheme of things. I measured 10 V out for 3.75 uA in at the 100 percent
gain setting. Thus, replacing the PMT with a photodiode would only
be useful for high intensity sources or low power lasers aimed directly
into the entrance slit. But that would be such a waste of the EP200
since one of its main benefits is having the super high sensitivity
detector built-in. In fact, when using the EP200 with coherent sources
like lasers, there may be a small amount of ripple in the peak response
versus wavelength and orientation of the instrument, presumably due to
interference effects similar to visible speckle. This is not present
with gas discharge sources. The EP200 is also polarization sensitive
with a difference in response of about 2:1 for s and p polarized light.
Thus, using the EP200's output to monitor the mode sweep of a random
polarized HeNe laser may result in excessive amplitude fluctuations.
But why would anyone want to do that with a monochromator?!

Detailed information on this series of instruments can be found
at Verity Instruments
Monochromators. A description and photo of the interior is there
as well as connector pinouts. One thing I did determine that isn't on the
Web site is that there is a small slide switch on the PMT HV PCB inside the
unit to select internal (adjustment pot) or external PMT high voltage control
on pin 1 of the DB9 (+2 to +10 VDC for -200 to -1,000 VDC). The Web site
simply mentions that internal or external HV control is selected at the time
the order is placed. Some units (don't know if they would be newer or older
than the ones I've seen) may only have a jumper. The lid can be removed
without much risk of contamination as the box is not sealed. Just don't
touch the diffraction grating as it cannot be cleaned.

A sticker under the micrometer cover as well as another one inside the unit
details the function of the 4 position DIP switch, which is to control the
PMT preamp bandwidth as follows (0=Off):

I think the HV will actually go down (up?) to 0 V but probably isn't very
useful much closer to 0 V than about -200 VDC. Note: Some documentation
I've seen shows the HV Programming input being -2 to -10 V but all the
units I have work fine with +2 to +10 V. I do not know for sure what
the DC offset pin is used for. It is connected to the wiper of the ZERO pot
and produces a small DC voltage (less tha 1 VDC) that varies with the pot.
But, it may also be an input to allow the controller to set the DC offset
remotely.

The next test was to look at the spectral lines of the discharge in the
bore of a 1 mW HeNe laser tube. Placing the bare tube next to the EP200
entrance slit and approximately level with it resulted in a large response.
(HV of -300 VDC and gain of 50 percent.) However, this particular EP200
came with 500 um slits, too wide for my taste. :) While the strong lines
could be seen, weaker ones adjacent to them were buried. :) So I modified
the slits by using 5 minute Epoxy to glue a piece of a single edge razor
blade to each, positioned to reduce the slit width to between 100 and 150 um
- about as narrow as could be done by eye. (See below for more details
on modifying the slit width.) This worked great in improving the
resolution and allowed weak lines adjacent to strong ones separated by well
under 1 nm to be resolved. However, since each slit was narrowed from one
side only (that was enough of a pain in itself!), the wavelength calibration
shifted by about 1 nm. (I must have guessed wrong since if they had been
narrowed from the proper side relative to each-other, the wavelength
calibration shouldn't have changed.) To remedy this, the micrometer
mounting plate screws were loosened just enough to allow the micrometer
to be nudged by about 1/1000th of an inch using the 585.25 nm and 587.56
nm yellow lines of the HeNe laser tube bore discharge as references, and
confirmed with the 632.8 nm red lasing line.

Those 585.25 nm and 587.56 nm lines are significant in that they are from
neon and helium, respectively, and if reasonably similar in amplitude, the
ratio of helium to neon is correct inside the tube. On the tube I tested,
the intensity of the He line was about double the Ne line, indicating that
the He:Ne ratio was high. That's better than the other way around. :)
This thing makes such determination so easy. :)

I've built a DC power supply and detector meter box (from junk parts of
course!) to drive the EP200 heads conveniently. By default, its analog
meter shows the detector output. However, by pressing a button, it will
show the HV, which is adjusted via a 10-turn lockable knob if the EP200
is set up for external HV programming.

I also constructed my own fiberoptic cable adapter (these are also available
from Verity) which positions the tip of an SMA fiber connector at the
entrance slit. The other end of the fiber would then have a focusing
lens that can be positioned conveniently near the spectral source. Even
though the amount of light coupled through the fiber into the monochronometer
is generally quite small for anything but a laser, with the high sensitivity
of the EP200, there is easily enough to take readings.

A second EP200 I acquired had a bad photomultiplier tube but I replaced the
original (Hamamatsu R928HA
Hamamatsu R928
Datasheet) with an RCA 931A I had laying around. That
seems to work OK, certainly well enough for my needs - it's way too sensitive!
There are probably many other compatible PMTs. As long as the PMT has the same
side-input, pinout, and fits the socket and housing, that's probably good
enough for non-critical uses of the EP200.

On a third EP200, the grating had fallen out of its mount. How the set screw
loosened up will probably remain a mystery. Although there is a long
scratch on the grating (possibly from the trauma, possibly from bouncing
around during shipping, or possibly it was there even when new). Since the
scratch just happens to be perpendicular to the rulings, it really doesn't
cause any degradation in performance of any consequence. This unit worked
fine after reinstalling the grating and calibration.

On a forth EP200, the PMT had actually cracked - the main cover had a
major ding in exactly the wrong place. Although no internal damage was
visible, this must have whacked the PMT. It's otherwise in good condition
awaiting a transplant.

Yet another one had a bad PMT (very high dark current or something) AND a
bad PMT transconductance preamp op-amp. The main problem replacing it was
that the space is very limited. It's also the only IC that's not socketed.
An LM741H in a TO-99 can is pin compatible, but with some creative lead
bending, a jelly bean LM741CN DIP (which is much less expensive) can be made
to fit. The original very high quality OPA111AH op-amp from Burr Brown (now
TI) is quite expensive, but the 741 should work well enough for measuring
wavelengths since long term stability is not required. Almost any other
common op-amp could probably be shoe-horned in its place. Pin 2 is the
input and pin 3 comes from the Zero Offset trim-pot on the front panel.
The op-amp offset trim pins are not used.

CAUTION: Do NOT attempt to measure the output of even a very low power laser
by aiming it into the entrance slit. That will completely overload the system
and may damage the photomultiplier tube. For a typical 1 mW
laser, just arranging the beam to hit a white card positioned at 45 degrees
near the entrance slit will provide more than enough signal with the PMT
HV near the lower end of its useful range (say -250 to -300 VDC).

CAUTION: Do NOT attempt to adjust calibration unless you have a source of a
known unambiguous wavelength to use as a reference! It's way too easy to
shift it too far to get back easily without one. A low power HeNe laser
or green DPSS module shining on a white card would be suitable, but
NOT a red pointer or other diode laser unless its peak wavelength is known
exactly.

Checking one of these units for basic functionality is quite easy,
requiring only a spectral test source like a neon lamp, HeNe laser tube,
or other gas discharge lamp. Given the extremely high sensitivity
of the EP200, using the light shining through the window from an
outdoor high intensity sodium or mercury vapor street lamp may even
be possible.

Additional equipment that will be needed are regulated +15 VDC and -15 VDC
power supplies (50 and 225 mA maximum, respectively), a 10K ohm pot and 5K ohm
resistor to build a circuit for setting the high voltage (HV) if using
external HV Programming, and a DC voltmeter capable of reading 10 V full
scale. A flashlight may be useful as a quick test for confirming that
the photomultiplier tube (PMT) detector is responding to light.

Referring to the pinout for the DB9 interface connector, above,
wire up the 15 VDC power supplies, Signal output, and HV programming
(just in case). Circuit Ground is the common for everything including
all voltage measurements. The COM test point is connected to Circuit
Ground. The 5K resistor goes to +15 VDC and then to the top (clockwise
end) of the pot, the wiper goes to the HV Programming input, and the
bottom (counterclockwise end) goes to Circuit Ground. The value of the
resistor and pot aren't critical as long as their ratio is 1:2 so that
the HV Programming input is 0 to +10 VDC. Anything from 1K to 50K should
be fine for the pot. For a permanent setup, a 10 turn pot may be desirable
as the gain is quite sensitive to HV.

There is no need to remove the large cover which encloses the optics and
electronics of the EP200 except to flip the PMT HV selector switch if needed,
or a major problem is found. For all tests, this cover should be in place
with the screws tightly secured. Ample details on what's in there can be
found on the Verity Web site or from the photo Verity
EP200 Monochromator/Detector Organization.

However, we all know that curiosity will get the better of you, so as long
as it's open, check that nothing has fallen out, that the diffraction grating
is secure in its clamped mount, it rotates freely against spring tension,
and that the metal shroud surrounding the grating glass itself is pressed
in as far as it will go. DO NOT touch the surface of the grating as it
can't be cleaned without degrading its performance!!! DO NOT attempt to
remove or adjust the grating in its mount - that affects focus and precise
wavelength calibration (beyond what is discussed below). It's not supposed
to be all the way in. If for some obscure reason it must be removed,
use a depth gauge or other instrument to determine exactly how far in it
should go and note which side is up (so that the blaze angle is correct
when reinstalled). If there are any serious dents in the cover, confirm
that there is no corresponding internal damage.

CAUTION: On all the EP200s I've tested, simply removing and replacing the
cover may alter wavelength calibration by a fraction of a nm. I assume that
any slight change in stress on the baseplate deforms it enough to shift
the wavelength peak. So, expect to have to do the basic wavelength
calibration described below if you do go inside.

Now that that's out of the way, block the entrance slit with a piece of black
tape and double check your wiring before proceeding. It doesn't matter if
the micrometer compartment cover plate (two thumbscrews) is installed for
any of these tests.

Connect your DC voltmeter or a multimeter between the HV monitor test
point or pin 4 on the DB9 and Circuit Ground or COM.

If there is between -2 VDC and -10 VDC present, the internal HV pot is
in control. Set it for -3.0 VDC corresponding to -300 VDC, which
is close to the minimum useful HV.

If there is no voltage present, adjust your HV Programming pot to
confirm that it is in control. If so, set it for about -3.0 VDC output
on the HV test point (-300 VDC to the PMT). Note: According to some
documentation I've seen, the HV Programming input should be negative.
However, all the units I have work fine with positive HV Programming
input. But if for some reason yours doesn't, flip the polarity and
try again.

If it is not possible to obtain any high voltage or the full -10 VDC (-1000
VDC to the PMT) on the HV test point with either the EP200 pot or external
HV control, the HV power supply (a potted brick, an EMCO model 6858) or
associated circuitry may be defective, or there may be a short in the PMT
or its wiring.

To change from internal to external HV programming or vice-versa, remove
the main cover on the EP200 by taking out all the screws around its periphery
and locate the HV select slide switch near the edge of the
electronics PCB between the PMT housing and slit mounting plate. Flip
it to the opposite position (toward the control/connector panel to select
the EP200 HV pot).

However, I highly recommend using external HV control as it is more flexible
and convenient allowing for very quick and easy control of PMT gain,
and won't wear out the internal pot! This is very likely the default
for most of these units.

Once the presence of HV has been confirmed and it is set for about
-300 VDC, connect the Signal test point or Signal Output (pin 5 on the
DB9) to your voltmeter. The reading could be anywhere from negative to
off scale at this point.

Set the 4 DIP switches to the OFF position to select maximum signal
bandwidth.

Press and hold the "GAIN" pushbutton and adjust the GAIN pot for
approximately +5 VDC (on the Signal test point or pin 5 of the DB9)
which corresponds to 50 percent gain. (The useful range is from 0.1 to 10.)

With no light input, it should be possible to set the ZERO pot to produce
near 0 VDC on the meter, though it may be quite touchy and like to go way
negative. Keep it just positive if it won't cooperate by going to exactly
0 VDC. If the Signal Output remains pegged beyond +10 V and won't settle down
in positive territory, try turning down the HV to 0 VDC. If the meter can
now be zeroed but pegs to over +10 V with almost any HV, the PMT is probably
defective. Else, it may be a circuit problem. (Some dark current which
varies with HV may be normal but with -200 to -300 VDC, it should be
negligible.)

Set the wavelength micrometer for approximately 600 nm. (If the
micrometer doesn't turn easily, check the black locking ring near its base,
counterclockwise a fraction of a turn to unlock.)
Each turn of the micrometer corresponds to 25 nm with each division being
1 nm. Multiply the direct reading by 100 to obtain the actual wavelength.

Shine a flashlight (if available) head-on into the entrance slit. With a
broadband source, there should be a response on the meter regardless of
wavelength. However, some LED flashlights may not he broadband. So, if
using an LED flashlight, rotate the micrometer through the visible range
and look for a response. If this test is successful, the EP200 is likely
fully functional but may need to be calibrated for wavelength.

Before proceeding with wavelength checks or calibration, the EP200 main cover
should be securely screwed down including the locking screw of the input slit
since a slight shift in wavelength may occur when this is done. The EP200
should be placed on a solid surface so it can't wobble as any position or
orientation change may result in a variation in the signal output and
confusion as to where a peak is located.

Set up your spectral test source in front of the entrance slit, or aim
the monochromator to the source. the f/3.5 acceptance of the system is low
enough that precise alignment isn't needed but arranging it to be fairly
close to in-line with the slit will be best.

CAUTION: DO NOT shine a laser beam directly into the entrance slit. Almost
any laser - even a half dead laser pointer - is way too powerful to be used
directly. However, it is safe to shine a low power laser on a white card
placed near the entrance slit. More on this below.

Refer to the appropriate spectral charts to determine what wavelength
emission lines should be present with your source. For example, three
relevant lines for the bore discharge of a HeNe laser are: 585.25 nm
(neon yellow), 587.56 nm (helium yellow), and 640.2 nm (neon red). The
normal HeNe red 632.8 nm is also present, but quite weak. There are
dozens of other lines present but the three cited are particularly
strong. If these can be located exactly where they should be, you're
done. If they are there but not exactly where they should be, you're
almost done. :) When turning the micrometer, avoid applying side-pressure
as this will result in a wavelength shift and hysteresis.

Note that with the typical 500 um slits, it is difficult to resolve the
585.25 nm and 587.56 nm lines as their spacing is less than the
spec'd resolution of the EP200. But it's no problem with 200 um
or narrower slits.

If you're not using a HeNe laser tube but a source like a gas discharge
spectral lamp containing some other gas(es),
spectra for varioue elements and compounds can be easily found by searching
the Web. The NIST Atomic
Spectra Database has an applet which will generate a table or plot of
more spectral lines than you could ever want.

If the spectral lines located above are at the proper locations on
the micrometer or within the uncertainly due to what minimal backlash there
is, you're done. Otherwise, adjustment of the micrometer
assembly will be required. This is best done with a HeNe laser or other
source with a single spectral line. Otherwise, it may be difficult to know
which line you're seeing. The use of such a laser is assumed below. Note
that a red diode laser pointer is NOT suitable as its wavelength is not
known precisely, nor is the line very narrow. However, a green DPSS laser
pointer at 532 nm is perfectly fine.

Set up a low power HeNe laser with its beam shining on a white
card placed at a 45 degree angle next to the entrance slit, approximately
centered and level with it. CAUTION: Do NOT shine the laser into the
entrance slit as it will be way too powerful. This would grossly
overload the system and may damage the PMT.

Attempt to locate the 632.8 nm wavelength (assuming a red HeNe laser)
by adjusting the micrometer. Unless someone has totally disassembled the
EP200 at some point, it should appear somewhere within the adjustment
range of the micrometer, probably quite close to where it should be. A HV
setting of -300 V and gain setting of 5 V (50 percent) should result in
adequate response for a 1 or 2 mW laser. You can try increasing the
HV slightly if the response is very weak so that the reading reaches
at least half scale.

If the wavelength location is too low, the micrometer must
be moved toward the entrance slit side of the unit. If too high, it
needs to move away. The return spring on the grating tends to push it away
but with the cover in place and pressing on the seals, some persuasion
may be needed.

The following three steps can probably be skipped if the wavelength is within
a few nm of the correct location.

Use a right-angle hex wrench to loosen both of the micrometer plate
mounting screws inside the micrometer compartment just so they are barely
touching the plate. The return spring will move the micrometer assembly
away from the entrance slit side of the EP200 to its stop.

Set the micrometer to 632.8 nm exactly.

Use a narrow screwdriver or other suitable tool as a lever vertically
between the micrometer mounting plate and EP200 main cover. Slowly move
the micrometer assembly toward the entrance slit side of the unit (pull
the tool toward the control/connector panel) while monitoring the Signal
Output test point or connector pin. When the position coincides with
632.8 nm, the reading will jump. With care, it should be possible to
lever the plate back and forth very slowly around this point stopping
exactly at peak output so the mounting screws can be tightened.
1/1000th of an inch movement corresponds to 1 nm. With care, a precision
of 1/10,000th of an inch is possible. This sounds more daunting than it
really is.

Check the peak setting. If the wavelength is only slightly off, it may be
possible to loosen the micrometer mounting screws so they are just snug
enough to hold the plate in position. Then, gently nudge the plate in the
appropriate direction to center the peak at the 632.8 nm setting.

If possible, check some of the lines in the HeNe tube discharge spectrum
to confirm calibration. If fine adjustment is needed, it's better to use
one of those because there are negligible interference effects to confuse
the peak location, as there are with the coherent laser output, which may
be somewhat touchy and dependent on input position and orientation.

If another laser is available with a much different wavelength (like
532 nm), it would be worth checking that it too is now lined up correctly
with the correct micrometer setting. Should this for some reason be off by
more than a fraction of a nm, the relation of the grating shaft to its
lever arm has changed. Adjustment of that is a more involved process
reserved for the advanced course. :) But, no one should have touched
and unless the unit was terribly abused, it shouldn't have changed on
its own.

This description probably makes the procedure sound like it will take all
day. Wavelength calibration should require only a few minutes unless you
are an absolute perfectionist, in which case it will take forever. :)

For end-point detection in whatever processes these instruments
normally monitor, the most common 500 um slit is perfectly adequate. But for
looking at closely spaced spectral lines, a narrower slit is almost
essential. Although the slits width isn't adjustable on the EP200,
modifying the slits to be 100 to 200 um is relatively easy. The first
procedure is reversible:

Remove both slit assemblies (slit plate in plastic holder). The entrance
slit is locked into position by the cover screw next to it. The slit
assembly can then be pulled out. The exit slit is under the circular cover
plate held in place by 4 small screws on the bottom of the EP200. Once the
cover plate is removed, the slit assembly can be pulled out. The entrance
and exit slit assemblies are identical.

Gently confirm that the thin metal slit plate itself is secure in each
plastic holder. On some units, I've found that the glue has weakened and
almost any pressure results in the slit plate popping out. If loose,
fallen out, or in doubt, use a tiny dab of 5 minute Epoxy to secure it.
Make sure it is parallel to the edge with the shiny side should be down

Prepare a sacrificial single edge razor blade. It doesn't have to be
new but the edge should be free of any nicks or dings. Use a pair of pliers
to break off pieces that will fit in the hole in the plastic behind the
slit.

Narrowing the slits from both sides is not essential but is desirable
to avoid a wavelength shift depending on where an small diameter input
source is located - above, below, or centered on the slit. This is because
the blades on each side will be at the different distances from the
diffraction grating. This may result in
an error of a fraction of a nm depending on source location, though
resolution will not be affected significantly. But, to prevent a fixed
wavelength calibration shift if doing only one-side surgery, the entrance
slit should be narrowed from the opposite side as the exit slit.

Put a tiny dab of 5 minute Epoxy in the recess near the edge on the
appropriate side and use tweezers to place the bit of razor blade in
position. I just eyeballed the slit width but using something more
sophisticated to measure it is permissible. :)

Check the status of each slit as the adhesive cures to make sure it hasn't
shifted position.

Reinstall the slits and check calibration. Since the slits are much
narrower than before, increasing the HV and/or gain may be necessary.
If the calibration did change significantly, use the procedure in the
previous section to correct it. With the narrower slits, calibration
error will be more noticeable.

Where it is known that going back to the wider slit will never be desired,
then the following may result in better performance:

Carefully pop the original metal slit insert off of the plastic holder.

Use a sharp pair of scissors to split the slit into two parts without
harming the slit edges.

File each side to the two halves can be mounted at the appropriate
closer distance.

Use 5 minute Epoxy to reattach them to the plastic holder confirming
the correct spacing and adjusting as needed before the adhesive sets.

The focus is more critical with narrower slits. If the diffraction
grating's position in its mounting clamp isn't exactly right, resolution
will suffer. If it's never been touched, then don't touch it now as
it's unlikely to have moved on its own, and wavelength calibration may
be affected by position. But, if you've been fiddling with the grating,
now is the time to adjust focus using a diverging beam (laser or LED) as
the input. Since this has to be done with the cover removed, the source
will need to be bright with gain set low enough so the ambient light
doesn't overwhelm the system.

These consist of a "head" unit which is a compact monochromator where the
diffraction grating rotates continuously on a motor shaft. With a suitable
controller, an optical spectrum over a wide range (e.g., from 300 to 1,100
nm) can be acquired in about 85 ms. Thus one application is in a fast,
though not particularly high resolution, optical spectrum analyzer. The
resolution is about 1.4 nm for my particular unit.

Simply applying 15 VAC to the power jack will make the motor spin,
though the stability may not be that great. Although motor speed is
regulated based on a voltage-to-frequency function from the 36
pulse/rotation optical encoder disc, it's likely intended to be
more precisely phase locked to a crystal reference by the controller.
The 6100 has a built-in trans-impedance (current to voltage) preamp
for a photodiode. If displaying the spectrum of a laser, the
photodiode can be almost anything as long as it's relatively small
area (low capacitance). I used one from a barcode scanner for
testing, just positioning it near the output slit. The 6100 provides
a trigger signal that can sync an oscilloscope which can
then be used to display the spectrum in leu of the controller and data
acquisition system. Although a digital storage scope is desirable, my
Tek 465B worked just fine in showing the 3 lines of my funny
yellow-orange-orange PMS/REO LHYR-0100M HeNe laser head. However, the
resolution is orders of magnitude poorer than would be required to view
the individual longitudinal modes of any HeNe laser, as wtih a
Scanning Fabry-Perot Interferometer (SFPI). The main
problem was excessive sensitivity. My photodiode detector had no gain
control, being, well, just a photodiode. Perhaps the intended
detectors have an adjustment. I had to use neutral density filters to
reduce the intensity to a level that didn't saturate the preamp.

If anyone has more specific information including schematics for
the 6105 head unit, or has Monolight hardware they'd be willing to contribute
to the cause, please contact me via the
Sci.Electronics.Repair FAQ
Email Links Page. I had to add a pot to adjust the pullup resistance
on the optical encoder on my sample as the signal was about half the
amplitude it should have been leading to some peculiar behavior.
Although it appears as though this pullup is
a "select on final test" resistor, being in a two terminal header rather than
being soldered into the PCB, I assume that somehow the output has gone down
due to a weak LED or other problem in the opto-detector reading the disc
pulses by reflection, similar to the reel rotation sensors in some VCRs.
It is probably a standard 3 terminal device (LED and photodiode) which
could be replaced but I can't read the part number without disassembling
the unit.

Optical Wavelength Meters

While instruments like monochromators and optical spectrum analyzers
are capable of determining the wavelength of light sources from light
bulbs to lasers, their accuracy depends on the precision of multiple
mechanical parts and the quality of the initial calibration. This is
because they use what might be termed an indirect
method of analysis - typically a diffraction grating moved by a precision
mechanism. If there is any real-time reference, it is likely only at
a single wavelength so there could be significant error at wavelengths
not close to it.

Where the source to be measured is broadband or has multiple spectral
lines, such techniques are generally the easiest and fastest (but see below).
However, where a single wavelength CW laser's output needs to be determined
very precisely, alternative methods are generally used. (Wavelength
meters capable of reading pulsed lasers also exist. See the section:
Pulsed Wavemeters.) Instruments
of this type may have an
accuracy and resolution that is orders of magnitude higher than
a monochromator or optical spectrum analyzer.

The following description is for one common approach and the one used in
the Burleigh WA-20 a typical older model dating to the early 1980s.

The basic wavelength meter or "wavemeter" compares the unknown input with a
reference laser by counting fringes for both sources simultaneously in a
Michelson interferometer where the path length difference is varied
periodically by a motor-driven mechanism. The unknown wavelength (or
frequency) is then related to the reference by the ratio of the number of
fringes for each during a fixed period chosen to be near where the
path length difference is small (to minimize the effects of the coherence
length of the unknown laser). Where the desired display is in wavelength
(e.g., nm), the reference wavelength is divided by N/No (where N is the
number of fringes for the unknown and No is the number of fringes for the
reference). Where the desired display is in frequency (cm-1),
the reference frequency is multiplied by N/No. (Frequency here is the
meaning used by spectroscopy-types: 1 cm/wavelength, or wave number.) The
division and multiplication can be easily accomplished with digital counters
and simple control logic, similar to that in any vanilla-flavored electronic
counter/timer. Phase-Locked Loops (PLLs) will generally be used for both
the reference and unknown detector signals to multiply the fringe counts
and thus the resolution.

A red HeNe laser is generally used for the reference laser. For the
Burleigh WA-20, an Aerotech OEM1P laser head with an Aerotech brick
power supply was found in one unit I checked. (I don't know if all WA-20s
used the same laser.) The brick ran from 12 VDC (it's
labeled 10-14 VDC) and had a fixed output of 4 mA with a compliance
range of 1,200-2,000 V. The specs of the OEM1P laser
head (from an old Aerotech brochure) are:

I doubt that any of these specifications are really critical. An output
power between 0.5 and 2 mW should be acceptable (and where a replacement laser
head is not electrically compatible, another HeNe laser power supply can be
easily substituted). Outside this output power range,
the signal processing circuitry may become unhappy and there are no
electrical adjustments. So, there's probably
little point in trying to retrofit a 10 mW REF laser even if it can
be made to fit (poking out the side of the case!). In fact, I've seen
an ND filter installed as original equipment to cut an overly lively
REF laser down to size! :) Even the fact that the OEM1P is linearly
polarized probably doesn't make a lot of difference, though a random
polarized laser might result in a larger variation of REF signal
amplitude during mode sweep. However, one thing that would make a
very slight difference is the isotope ratio of neon
in the gas-fill, which can shift the
peak of the neon gain curve by almost 1 GHz or 1.4 pm, more than
1 count in the LSD of the WA-20 display. This in addition to the
contribution of REF frequency change due to mode sweep. The hardest
part may be in mounting a non-exact replacement laser - there are
a pair of narrowed sections on the Aerotech laser head which mate
with brackets in the WA-20! (Melles Griot may have a suitable
laser head that would be a drop in replacement.)

Even if not stabilized (but with a known gas-fill),
its wavelength will be accurate to better than 1 part in
106. A stabilized HeNe laser
locked to the gain curve can be a couple orders of magnitude better and an
iodine line stabilized HeNe laser, even better. (Some later Burleigh/EXFO
wavemeters do incorporate stabilized HeNe lasers for the reference.)
Another source of error is the change in the refractive index of air
over the typical wavelength range of at least 400 to 1,000 nm. For this
reason, for better accuracy, some wavelength meters put the interferometer
optics in a vacuum chamber (less than 10 Torr). However,
simply providing a lookup table for wavelength correction would be
nearly as effective and much less of an implementation issue, though
the actual pressure and temperature have to be taken into consideration.

The performance of this fundamentally simple device is quite
amazing. The resolution and accuracy of the Burleigh WA-20, which
is one of the earliest commercial wavemeters, is better than
1 part in 106 (less than 0.001 nm or 1 picometer over the
measurement range of 400 to 1,000 nm!). No routine calibration is required.
While degradation in alignment is possible, the effect
will be to increase the power level needed to take a reading but
will not noticeably effect the resolution and accuracy. As long as the
instrument is happy with the signal levels, the resulting display
should be accurate. The most common problem may be a bad belt between
the motor and interferometer drive! And an elastic band or tape player
belt will work just fine, thank you. :-) (But really old belts may decay
into a gooey black mess.)

In fact, the modern replacements for the WA-20 are the Burleigh/EXFO
WA-1000 and WA-1500. (Go to EXFO,
"Products and Solutions", "Spectral Test Equipment", "WA-1500 and WA-1000
Wavemeter", or search for "WA-1500".) The WA-1500 is virtually identical
mechanically to the WA-20
but incorporates a frequency stabilized HeNe laser for the reference and
keeps the interferometer at atmospheric pressure instead of in a vacuum.
Software correction for the non-linearity of the index of refraction of
air is then used with inputs from pressure and temperature sensors. With
its stabilized reference laser, the accuracy of the WA-1500 is better than
that of the WA-20 - +/-0.2 picometer compared to +/-1 pm for the WA-20.
The WA-1000 uses a non-stabilized HeNe laser like the WA-20 and has
similar accuracy.

A note to those out there who believe in running wavemeters continuously
because they believe this results in better performance: There is no need.
The performance of wavemeters like the WA-20 or WA-1000 is
essentially the same as soon as the reference laser turns on as if run
for a year. There is nothing to warm up that matters. So, save the
reference laser and mechanics (where appropriate) and turn your wavemeters
off when not being used! This also applies to those like the WA-1500 that
incorporate stabilized HeNe lasers for the reference except that a warmup
period of 10 to 20 minutes is required for the laser to lock. But after
that, the wavelength drifts at most by much less than 1 count in the
last digit (0.1 picometer).

It should be noted that this implementation of a wavemeter is a subset of
a more general technique called Fourier Transformer Spectroscopy which is
capable of dealing with arbitrary spectra. (See, for example:
World of Physics: Fourier Transform Spectrometer.) Rather than
simply counting fringes, the Fourier transform is taken of the fringe waveform
during one or more scans of the path length difference. For a
single spectral peak as with a CW single frequency laser, the FT is a
single peak. For a source with multiple peaks, the fringe pattern becomes
visually complex, but the Fourier Transform will be the desired spectrum.
This approach is also used in some wavemeters that can deal with multi-line
laser input. For example, the WA-650 is an add-on that converts the WA-1500
or WA-1000 into an optical spectrum analyzer by Fourier Transform processing
of the fringe pattern.

In fact, it should be possible to process signals from the back of almost any
wavemeter which has a built-in reference laser to use it as an optical
spectrum analyzer. The interference signal for the unknown source, the
interference signal for the reference laser, and a scan position sync
pulse are required. This would be very simple if the scan was linear. But
with wavemeters using a motor-driven scan like the WA-20 or WA-2100, the
speed and thus fringe frequency isn't perfectly constant and this would
totally mess up the FFT. It should be possible to correct it as long as
a reference laser signal is available. The details are left as an exercise
for the student. In fact, this would make a nice term project in DSP. :)

But, the beauty of the basic single wavelength wavemeter is at
least in part due to the simplicity in terms of its principles of
operation, mechanical construction, and electronics.

While a Scanning Fabry-Perot Interferometer (SFPI) may have better resolution,
it typically doesn't have very good accuracy or stability with respect to
absolute wavelength or frequency unless additional techniques are used,
adding to complexity.

While minor enhancements like the use of a voice coil magnetic drive instead
of a motor can improve the speed and reduce the size of the Michelson
interferometer-based wavemeter, higher performance instruments may use
something called a Fitzeau interferometer with no moving parts. Multiple
wedged etalons generate fringe patterns which depend on the source wavelength.
These are captured via CCD arrays and analyzed in software. These instruments
can deal with pulsed lasers and have much faster dipslay rates (100s of Hz or
more compared to a few Hz for motor driven interferometers) and even more
immune to alignment problems.

The Burleigh WA-20 is a typical older wavelength meter that uses a motor-driven
moving interferometer mirror and fringe counting to determine wavelength
(in um) or inverse frequency (in cm-1) of CW lasers between
0.4 and 1.0 um for the visible, which may be extended to 4.0 um with
the IR option (which substitutes a different beamsplitter and detector).
This model dates from the early 1980s, though the specific unit I'm working
on has a manufacturing date of 1995. There was also a WA-10, with the
only difference being that while the
WA-20 maintains the entire interferometer inside a chamber that can
be evacuated to below 10 Torr to for better accuracy, the WA-10 simply
has a dust cover. The reason that a vacuum is beneficial is that there
is a small non-linear depedence of the index of refraction of air on
wavelength so a measurement of a laser with a wavelength far away from
the 633 nm reference might see an error of as much as 3 parts in
106 in air. If the temperature, pressure, and humidity
are known, a lookup table can be used to eliminate the error, but
that's probably more trouble than it is worth. However, the nice thing about
the WA-10 is that it's a lot easier to work on it when doing alignment not
having to deal with the vacuum chamber and vacuum-tight covers over the
mirrors! And it weighs less. :)

A red HeNe laser (polarized but not stabilized) is used as both the wavelength
reference, and to provide a "tracer" beam to facilitate alignment of the
unknown laser to the input of the WA-20. It passes through the same
interferometer optics, but more-or-less in reverse. Thus, alignment
of the reference laser beam is sufficient to guarantee alignment of
the entire system.

Neither the WA-10 or WA-20 are manufactured or supported now, but
the modern replacements, the WA-1500 (with stabilized HeNe laser reference)
and WA-1000 (without) are substantially similar in design, though they
both operate without a vacuum, but have pressure and temperature sensors using
software correction for the non-linear index of refraction of air. With its
stabilized reference laser, the WA-1500 has somewhat better accuracy
than the WA-20 while the WA-1000 is similar. They both have better
sensitivity (20 uW instead of 100 uW). For specifications,
Go to EXFO, "Products and
Solutions", "Spectral Test Equipment", "WA-1500 and WA-1000 Wavemeter".

The WA-20 I had on loan for testing and adjustment was so misligned when
I received it that the tracer beam was partially cut off and less than
1/10th the intensity it should have been, and the "Ref Error" light was
flashing due to low signal level. Yet, despite these problems, it was
still able to measure the wavelength of an external red HeNe laser to
the expected accuracy, though higher than spec'd power was required.
That is, after the drive belt which had fallen off was put back in place. :)

There is an alignment procedure in the user manual that is strightforeward,
if somewhat tedious. It uses the reference laser entirely to align the
mirrors and beamsplitter in relation to the input aperture. Once this is
done, the unknown laser input is also automagically aligned since it uses
the same optics. Once this procedure was complete, the system was able
to read the red HeNe laser as well as a highly attenuated C315M green
(532 nm) laser at power levels below the spec'd minimum of 100 uW.

Additional items that still require attention are obtaining a replacement
drive belt and replacing the O-ring in the motor vacuum seal feed-through since
it's leaking at too high a rate. However, except for being run at
1 atm instead of a vacuum and accepting the slight reduction in accuracy,
it's now in good shape.

And, the Power indicator uses a strange 60 V, 20 mA incandescent lamp, and
of course is likely to be burnt out on a well-used WA-20. I replaced it
with a high brightness LED, soldered to the slide contacts of the original
lamp along with a 1N4148 across it for reverse polarity protection. An
additional 3.9K, 2 W resistor and 1N4007 diode were added in series with
the original 3.9K resistor in series with the one already there. An
alternative that wastes less power would be to tap off one of the
5 VDC or 12 VDC supplies but this makes it easy to restore the original
arrangement if desired.

The WA-2500 is appropriate named "WavemeterJr" as it is a much
smaller, lighter, and simpler instrument than the WA-20 which may be used
to measure wavelengths from 400 to 1,800 nm using separate detectors for
VIS (400 to 1,100 nm) and IR (up to 1,800 nm). The
precision is lower (5 digits instead of 6 digits) and it lacks the built-in
reference HeNe laser, so an external HeNe laser must be used for calibration.
However, this can be done regardless of whether the instrument is set for
VIS or IR as the IR photodiode still has decent sensitivity at 633 nm.
Given the lower resolution, this isn't nearly as important as with
the WA-20. But the WA-2500 also lacks the tracer beam, and in fact, only
allows for fiber coupled input via an FC connector on the rear panel.
It is microprocessor-controlled and includes an RS232 port
for data collection. A Web search for "Burleigh WA-2500" will locate
an operation manual with description, specifications, and alignment
information.

Like the WA-20, the WA-2500 is based on a Michelson interferometer, but
the optical setup is much simpler. The main component
is a single Cube-Corner (CC) on frictionless dual flexure mounts with a
electromagnet ("voice coil") to "excite" it at its mechanical resonance of
about 10 Hz, providing 10 readings/second (or 1/s if averaging is turned on).
An automatic locking mechanism keeps it from bouncing around when power is
off, but makes an annoying loud clunk in doing so. :) The other interferometer
optics include a 45 degree beam-splitter mirror, and two 0 degree mirrors.
(A detailed layout is in the manual.)
The alignment of one of these mirrors is critical, with adjustments accessible
from the back panel. I just wish they had used a higher quality mount with
finer pitch screws! There is also a fixed (45 degree) fold mirror which
simply directs the internal beam to the adjustable one on the backpanel
to make the optical layout work out in the available space.

The only time there's a need to go inside is to flip the detector board
to select VIS or IR. (One screw and one jumper.) A photo is shown
in Photodiode Preamp PCB from Burleigh WA-2500.
On this WA-2500, the final op-amp (LF347) was
found to be blown along with a toasted (but apparently still functional)
resistor. Someone may have plugged the cable in incorrectly, though this
would seem to be difficult. :) Or it may be that the "Monitor" BNC
on the rear panel is in parallel with this output, so perhaps someone plugged
something in there that shouldn't have been plugged in there. :)
So, the signal level was very low almost
never showing up on the bar-graph display and only a few hundred mV at
most from the Monitor BNC, with the machine either producing Lo Signal or
Alignment Error. There was also no Window signal (middle BNC) at all
which initially led me to suspect there might have been logic problems.
But apparently, that isn't generated until some minimum signal level is
detected. Once the op-amp was replaced, it instantly sprang to life.
However, for optimum performance, using the identical op-amp (or at least
one with adequate bandwidth) for that stage at least seems to be critical.
Substituting an OP27 in place of the LF347 (which was the only single
op-amp I had available at the time) resulted in a non-uniform
signal envelope and incorrect calibration with respect to Hi Sig and the
bargraph as well as limiting the range of the bargraph. With correct
op-amp, the trim-pot can be adjusted for a reasonably flat signal
envelope.

The tuneup then consisted simply of peaking the signal level using the
mirror alignment screws.

The WA-2500 works very well with single longitudinal mode (single frequency)
and multimode lasers where the modes stable and closely spaced, as they
are in all HeNe lasers. Ifthe bandwidth of the lasing modes is larger
than the coherence period over which the Mechelson interferometer samples,
the WA-2500 will reduce the resolution so that a meaningful measurement
can still be made. This is done by looking at the envelope of the fringe
signal and only sampling during a segment between where it goes to zero.
However, (not surprisingly) there can be real problems with multimode
lasers where the modes may be jumping around. I could not get reliable
measurements using a green laser pointer or crappy green DPSS laser module
(which is probably similar). But it works flawlessly with any HeNe and high
quality DPSS lasers like the Coherent C315M. There can also be some
loss of resolution if using a multi (spatial) mode fiber for the input
though this is usually minor, may be worth the greatly reduced hassle
in coupling to the large fiber core.

Performance is somewhat better in terms of sensitivity and consistency
with a single mode fiber having a core size appropriate for the laser
wavelength, but a multimode fiber can be used without too much difficulty.
Even the one Burleigh provided can only be truly single mode for the
longer wavelength range. A 9/125 telecom fiber will not be single
mode for a 532 nm green laser, which requires a 3 or 4 µm core to be
single mode inside the fiber.

Most of the above also applies to the WA-2200, of which there appears to be
no record on-line. The sample I tested did not allow for swapping between
VIS and IR, and the detector PCB was slightly larger. But otherwise,
the optical components and layout are identical to that of the WA-2500.
My unit had a broken glass Moire plate, probably from being dropped hard.
There are two glass plates with patterns of fine lines in close proximity
to generate the reference signal in lieu of the HeNe reference laser. But
the mass of the moving part of the optics could indeed cause them to
contact if the shock is severe enough, even when locked. I remounted
the remaining good piece as best I could - the other piece was no where
to be found, presumably lost by the previous owner. I believe there is
enough remaining to provide the necessary signal. I finally found the
PLL test-point. Here are all the test-points with their function:

I have no idea why there are two test-points that appear to have +5 VDC
on them, but perhaps one is a reference voltage or digital and analog,
or something. :) TP1 through TP5 are near the power supply; TP6 is on
the other side of the PCB near the front panel. There are 4 trimpots
on the main PCB:

Adjusting alignment of the two Moire plate varies the PLL signal amplitude
as expected. But the separation appears to have to be at the limit the
mounting screws will permit to get a clean signal.
With some fiddling, it was possible to get it up to almost
1.5 V p-p, a bit below the 2.0 V p-p the manual wants. But behavior is
essentiaally identical down to less than 0.5 V p-p so that's probably OK.
"PLL Err" is displayed on power-up a couple times but it then dissapears,
and that may be totally unrelated to signal level.

I also found that the pot on the photodiode preamp board changes its
bandwidth and setting it lower produces a flatter signal envelope at
the expense of some gain. I'm not sure that this WA-2200 is quite as
stable as the WA-2500 but it isn't too bad now. However, there is an
annoying intermittent problem: Occasionally, usually a few minutes after
being powered on, it will get into some state where it's not happy with
any signal level producing "Lo Sig" or "Hi Sig" or possibly other errors
continuously. This may continue for a few minutes and then totally
disappear. There's a slight possibility it has to do with back-reflections
destabilizing the test laser (Melles Griot 05-LHR-911) which is directly
fiber-coupled to the wavemeter without an optical isolator, but that's
not too likely.

The WA-1100 and WA-1600 are more modern Michelson interferometer-based
instruments. They are both smaller and lighter, and are
microprocessor-controlled along with environmental sensors providing
for wavelength compensation and eliminating the need for the massive
vacuum chamber of the WA-20. The primary functional difference between
the two models is that the WA-1100 uses a normal 1 mW polarized HeNe
laser head for the reference laser while the WA-1600 uses a frequency
stabilized HeNe laser. The use of the stabilized reference laser
allows for approximately a 5X increase accuracy and a 10X increase
in resolution. However, one cannot simply install a stabilized
HeNe in the WA-1100 to convert it to a WA-1600 as there
are other differences including the firmware and possibly the actual
interferometer as the update rate on the WA-1100 is 10/second compared
to only 1/second for the WA-1600. The WA-1100 and WA-1600 appear in some
ways to be cost/size/weight-reduced versions of the WA-1000 and WA-1500.
The input to both of these instruments is fiber-coupled, with an FC/APC
as the default connector. (There is no free-space option.)
Complete specifications for both
instruments are in the operation manual, easily found on-line by
searching for "Burleigh WA-1100 manual".

The interferometer in the WA-1100 has been greatly reduced in size and
complexity. (I have not seen the internal organs of a WA-1600.)
It uses a single retro-reflector (cube-corner) instead
of a dual-sided one as in the WA-10 and WA-20. It's on a much
much smaller motor driven linear slide, and
the remaining optics are installed in a compact single-piece precision
milled aluminum structure. There is still a rubber
belt but hopefully, it won't decay and require replacement like those in
the WA-10 and WA-20 as gaining access to the belt appears to be more
involved, buried beneath the optics. A single PCB (the Sensor Board)
mounts above the interferometer as shown in
Burleigh WA1100 Wavemeter Sensor Board.
There are 3 photodiodes (PDs) facing downward into it, one each for input
laser power, reference fringe detector, and signal fringe detector. Repair
would be more difficult as (1) most parts on the PCB are SMT and (2) it's not
possible to get to the optics with the PCB (and PDs) in place. However,
like the WavemeterJr, there really is only a single mirror that requires
alignment and those adjustments are accessible without disassembly.
That is accessible from the right side of the interferometer without
any further disassembly and consists of 2 kinematic adjustments screws
and locking set-screws near them. Peaking the reference fringe amplitude
should also result in optimal signal alignment.
Everything else is glued in place.

The reference lasers are typically OEM versions of the Melles Griot 05-LHP-491
for the WA-1100 and 05-STP-910 for the WA-1600, fiber-coupled directly to
the interferometer. The fibers are not connectorized, only terminated
and glued. The input fiber needs to be able to pass wavelengths up to
1,700 nm with minimal losses, so it is assumed to have a rather large core.
However, the reference fiber may be the same and thus multi-mode at 633 nm,
and it is sensitive to power changes with even modest bending radii or changes
in routing. There is a beam sampler and silicon photodiode that monitors
laser power before fiber coupling, which may be read from the
front panel, so it won't detect problems with the fiber.
There is nothing special about these particular model reference
lasers, but if the beam diameter and divergence of a replacement are not
close to the original, the coupling efficiency may be reduced unless
the focus position of the fiber (which is glued) at the laser end is
changed. The WA1100 in the photo actually has a JDSU 1107P installed
as the replacement reference laser. And it indeed appears to have significant
power coupled into the fiber cladding for this reason. Attempting to break
the glue bond was deemed to risky.

The specifications list a wavelength range of 700 to 1,700 nm. And indeed,
the lower bound seems to be quite strict. There is even a VIS-blocking
filter in the optical path, with the reflection from it going to the laser
power PD. It's not clear why the wavelength range doesn't extend further into
the visible. Certainly the IR PD still has some sensitivity at 633 nm
so it should be possible to measure the wavelength of a common HeNe like
its own reference! And for that matter, why not include a second signal
PD so that the measurement range could be extended down to 400 nm or beyond
as is done with the WavemeterJr? With minor modifications to the firmware,
it should not require much more than a dichroic (or even broad-band)
beam-splitter and $2 silicon PD. Darn Marketing! Cost reduced, strip
out useful features to promote sales of the higher priced spread. ;-)

The WA-7100 and WA7600 appear are geared to the telecom industry providing
spectral analysis capabilities using the same Michelson interferometer
technology as the WA-1100 and WA-1600. Rather than simply displaying the
precise wavelength of a single laser line, these instruments provide a
spectrum analyzer type display using the Fourier transform of the fringe
signal rather than simply counting fringes and comparing the count to
a reference laser or grating. In other fields, this would be called
a Fourier Transform Infra-Red Spectrometer or FTIR. ;-)

These are larger and heavier than the WA-1100 and WA-1600, probably
primarily to provide space for the larger LCD screen on the front panel.
But the interferometer "engine" appears to be similar or identical.
The microprocessor-based controller is physically similar but provides
much more sophisticated measurement and display capabilities. Similar
environmental sensors provide for wavelength compensation. As above,
the primary difference between the two models is that the WA-7100 uses
a normal 1 mW polarized HeNe laser head for the reference laser while
the WA-7600 uses a frequency stabilized HeNe laser. The use of the
stabilized reference laser allows for approximately a 5X increase
accuracy and a 10X increase in resolution. And, a stabilized laser
cannot simply be installed to increase resolution.
The input to both of these instruments is fiber-coupled, with an FC/APC
as the default connector. (There is no free-space option.)
Complete specifications for both
instruments are in the operation manual, easily found on-line by
searching for "Burleigh WA-7600 manual".

There are several subsystems inside the unit as shown in
Burleigh WA7600 Multi-Line Wavemeter Interior View:
DC power supply (upper right), fiber-coupled Melles Griot 05-STP-910 stabilized
reference laser with its HeNe laser power supply brick (right), interferometer
optics with processing PCB on top (covered, bottom), microcontroller PCB
(upper left), display PCB (attached to front panel, hidden), and a little
auxiliary PCB (assumed to be for the motor driver as with the WA-1100, left).

As noted, the interferometers in the WA-7100 and WA-7600 are probably similar
or identical to the ones in the WA-1100 and WA-1600, respectively,
though I have not removed the cover to inspect it. The reference lasers
are the same as well. See the previous section for more info.

While one of the systems described above is named "Wavemeter Junior", this one
should perhaps be called "Wavementer Lite". And it's not really a wavemeter
since it doesn't count fringes in any way, shape, or form using either an
actual laser or mechanical reference.
And there's no interferometer, so it really doesn't
compute wavelength directly. Rather, a diffraction
grating directs the incoming light from a 600 µm fiber onto
a Position Sensitive Detector (PSD). Analog circuitry generates
a voltage proportional to the wavelength
based on the centroid of the spot position. An A/D converts that to a
display of wavelength. The spec'd default wavelength range is
500 to 1,000 nm. The manual including specifications may be found at
WM4100
Wavelength Meter Operation Manual. (A Web search will also find
this manual.) Hamamatsu, one
supplier of PSDs, has an on-line "optics handbook" extensive technical info
on PSDs in Chapter 02: Silicon Photodiodes.
Or, for a quick intro, see
Wikipedia
Position Sensitive Device. The 1-D version would only require the
equation for X:

Ib-Ia
x = Kx * -------
Ib+Ia

where Ia and Ib are photocurrents from the ends of the PSD and Kx is a
scaling factor. The analog circuitry of the WM4100 implements this
equation directly using AD706 dual op-amps, PMI AMP03 differential
amplifiers, and an Analog Devices AD632 divider IC. However, since for the
diffraction grating, angle is a function of the arcsin of the
wavelength, and the spot is projected onto a flat surface, additional
calculations are required to correct for these non-linearities. Thus there
is an AD538 "Real-Time Analog Computational Unit (ACU)" involved somehow.
More on this when I figure it out. :)

Where incredible accuracy is not of key importance, this approach works
well. Any source with a single peak wavelength that can be coupled into the
input fiber will produce a reading and the response is a fraction of a
second - essentially the update rate of the A/D.
So, multi-longitudinal mode laser diodes as well as high
brightness LEDs will be acceptable.
If you can eyeball a peak wavelength using a spectrometer,
this thing will find it. More or less. :)

And it is relatively small and light weight!!!

I now have reverse engineered circuit diagrams for most of the system.

The unit I acquired appeared to work fine in general and displayed around
633 nm for a red HeNe. But a green (532 nm) laser pointer known to have
an effective IR-blocking filter produced 1,070 nm - around twice what it
should be. It turned out that the 2nd order green spot appeared at the
high end of the sensor rather than being blocked (or so I assumed) and
the 1st order spot was blocked, and that obviously some calibration would
be required. Someone *had* been inside as the internal ST cable at
the diffraction grating-end was only half pushed into the connector and
some mounting screws were loose. By slightly adjusting the angle of the
diffraction grating (which secured tightly), the green spot was easily
positioned in the Hopefully the 3 user adjustments will now suffice to
tune it up and they didn't also twiddle the 3 unmarked trim-pots!

However, upon further testing - after attempting to adjust the
grating and all of the pots (including those that shouldn't be twiddled),
I came to the conclusion that this instrument must have been set up for
something like 600 to 1,100 nm. There was no electrical or optical
adjustment that would ever result in a reading much below 600 on the display.
Indeed. the full model number is WM4100C. So, perhaps the "C"
means something. While an electronic failure could have resulted in a
bogus 100 nm offset messing up the readings, that wouldn't explain how the
2nd order 532 nm spot hit the PSD. The grating position didn't appear to have
been changed - at least its screw was very tight. No other optical
adjustment could have had such a large effect. The screws securing the
input lens were just snug, but that optic has very limited adjustment range.

At present it is awaiting final calibration, but does
read down to the HeNe 594.1 nm (yellow) line - barely.

Here are a couple of photos and more information on calibration adjustments.

Overall View of VERE WM4100 Wavemeter
shows the uncluttered front panel. The display of 894.4 is simply
where it seems to float with no input. The LED to the left of the
digits will turn on when there is sufficient input power for the
reading to be accurate. This may occur even before the analog meter
has moved detectably.

Interior View of VERE WM4100 Wavemeter shows
the interior with the approximate beam path of a 633 nm input beam. The
orientation is with the front at the left in the photo. The
fiber is a standard ST-ST patch cable with a 600 µm core. The PSD
is in the rectangular box at the upper right. I assume the 3 cables are
for bias, and the two segments of the silicon photodiode. The bulk of
the main PCB is taken up with the DC power supplies. The analog
circuitry is at the top. Only the 1st order diffracted beam path is
shown; the 0th order beam is blocked by the internal baffles while the
2nd order beam (if present) hits beyond the active area of the PSD
(to the left).

The only significant optical adjustment is the orientation of the diffraction
grating, set so the 1st order beam position on the PSD corresponds to a
bit more than the wavelength range.

There are a total of 8 electrical adjustments.

Three multi-turn trim-pots are considered user adjustments with calibration
instructions documented in the user manual:

RANGE (accessible from below, near the front) is essentially a
gain control for the output of the analog divider which sets the span for
the minimum and maximum wavelengths covered. It is visible standing up
near the top-middle of the photo.

Zero (accessible from below, toward middle) is used to equalize
the low and high power readings so they are the same. It is visible
standing up near the top-left of the photo.

OFFSET (accessible from rear) adjusts the wavelength reading
+/-10 or 20 counts. Its orange, yellow, and violet wires can be seen
at the upper-right of the photo.

Five single-turn trim-pots are used for factory calibration and are not
documented in the manual. There are visible in the photo while two others
are hidden by the analog meter and while capacitor. I have not fully
determined what they are for but here is what is known. Take this with
a very large grain of optical glass unless as noted!

The trim-pot in the top-middle fine tunes the relative gain of the
two sections of the PSD when the signal is above a certain threshold
(high range only, enabled by a relay). Below this point the gains are
equal, set by 402K ohm 1% resistors.

The trim-pot below it calibrates the wavelength monitor output from
the BNC on the rear panel and has no effect on anything else.

The trim-pot to the left of them next to the gold-covered IC may
have something to do with low end linearity. It connects between pins 17
and 18 of the AD538: "Real-Time Analog Computational Unit (ACU)". Putting
a variable resistance between these pins affects the gain of the "Log Ratio
Function".

The trim-pot hidden by the analog meter also doesn't appear to
do much.

The trim-pot hidden by the large white capacitor may be a master
offset.

Most of the previous descriptions are for older wavemeters that only
can be used with CW lasers since they are based on Michelson interferometer
with motor driven mirrors and require significant time to take a reading.
While these type of systems are still manufactured, modern high performance
wavemeters that can be used with both CW and pulsed lasers
are now fully "solid state" with no moving parts.
They typically employ one or more Fizeau interferometers and linear diode
(e.g., CCD) detectors. The interferograms (fancy name for fringe pattern
images) are then transferred via to a PC for analysis and display.
For more info, check out companies like
Bristol Instruments and
High Finesse GmbH.

Ring Laser Gyros

Mechanical gyroscopes measure rotation by measuring forces on a rotating
mass which has been machined and spun at high precision. Having moving
parts they are often clunky, bulky and distinctly 'low-tech'. They often
take a long time to 'spin up' and stabilize.

In principle the ring laser gyroscope can replace these with a fully
optical system using counter-rotating laser beams, photodetectors, and
digital electronics with no moving parts larger than photons and electrons.

In practice, it isn't so easy.

In its simplest form, the ring laser gyro (RLG) consists of a solid
triangular block of glass with a hold drilled out parallel to each edge.
Mirrors are added at each corner, a laser gain media such as Helium-Neon is
added and a stable laser cavity is established. The gain media is chosen
to allow two counter-rotating laser beams to be established - one clockwise
(CW) and the other counter-clockwise (CCW).

If the RLG is stationary (not rotating) with respect to its central
axis, the relative phase of the two beams is constant and the detector
output is constant.

If the RLG is rotated about its central axis, the CW and CCW beams will
experience a phase shift in the opposite direction. Since a ring laser
gyro is an active device (as opposed to a passive, fibre-optic gyro) this
change is phase will force a corresponding change in the optical frequency
of the counter-rotating beams.

At some point two of the output beams are mixed together and (due to the
slightly different optical frequencies) a beat frequency in the audio band
will result and this can be measured to very high accuracy with a
photosensor and appropriate electronics.

The frequency measured is known as the Sagnac frequency. It is
proportional to the rotational velocity, operating wavelength and ratio of
cavity area to perimeter.

A complete 3-axis inertial platform would require 3 RLGs mounted at 90
degrees to each-other. The entire affair can be fabricated inside a solid
glass block!

However, there are problems with this simplistic implementation.

To obtain a discernable signal, the laser must be operated in single
mode. This usually means the power level must be fractionally above the
threshold of lasing and this typically means very weak output beams.

If the rotational velocity is slow, the small amount of back-scattered
light due to imperfections in (even very high quality) mirrors causes the
two beams to phase lock together and there would be no output! I practice
this limits the minimum size of the laser, quality of the mirrors or
requires mechanical noise (dither) to be added in an attempt to force the
beams to be more independent.

Where high precision, and therefore large area is required the cost of

Fabricating a large monolithic slab of low thermal expansion glass is
extremely difficult. As a result, very large cavities have been constructed
in a discrete fashion, i.e., from individual pipes mounted on a concrete base.
But it is difficult to maintain mechanical stability of the device.

For the most part, these difficulties have been overcome to a degree
sufficient to allow for navigation of aircraft, spacecraft and submarines.
In such applications RLGs are increasingly being used in place of
mechanical gyroscopes.

Photos of an RLG laser assembly from a commercial inertial platform can be
found at Flavio's Ring
Laser Gyro Page. This is a triangular ring cavity
dual discharge HeNe laser with part of the beam path being external to the
block. One or more of the mirror mounts include coils, presumably to do
the dithering. (Hopefully, there will eventually be descriptions there as
well once we figure out what's going on.)

If you go to the
Laser Gyros
Directory, you'll find photographs of an early square He-Ne ring
laser gyro built by Sperry and some early designs for Honeywell's monolithic
ring laser gyros.

The Sperry gyro couldn't actually be rotated in the lab - kind of hard to
spin a one ton or thereabouts optical table. So they relied on the
Earth's rotation, or at least the vector component of it perpendicular
to the table at Sperry's latitude, to test their system.

I recall a conference talk on their work in which the speaker noted
that, given the backscattering and lock-up problems associated with a
ring laser at this low rotation rate, their primary conclusion was that
as best they could tell the Earth was still rotating, but at a highly
uncertain rate.

The mechanical precision is the hard part and that's what makes it virtually
impossible for an amateur to construct a ring laser gyro. The two opposite
traveling waves have to have extremely high spectral purity which translates
to high quality, high reflectance flats at the corners. Not a home job.

It might be easier to build a fiber gyro in which the light passes many
times around an effective ring through a wound fiber.

(From: Christopher R. Carlen (crobc@epix.net).)

The mechanical part is horrendous. We have an open cavity HeNe at my school's
lab, and it is a challenge to keep lasing on a heavy damped breadboard with
the mirrors mounted on a thick dovetail rail, bolted to the breadboard.

Then you complicate that by going from a straight, two-mirror cavity to a
three or four mirror cavity ring configuration, and then spin it real fast.
Can you say "centrifugal force?"

A fiber loop isn't quite the same as a ring laser, because the ring laser
actually has the laser gain medium in the ring. As opposed to having the beam
directed into a ring. The gain medium in the ring cavity ensures a standing
wave is set up in the cavity, which would not be so for the fiber loop.

Of interest for the future of laser gyros are the new photorefractive polymer
devices that exhibit the property of two-beam coupling. This device allows
coherent transfer of energy from one beam to another, when the beams are
intersected in the material. This can be used to assemble a ring resonant
cavity, pumped from the outside by a laser. This can be done with a small
diode laser resulting in an assembly much smaller and easier to keep still
while spinning than a gas laser ring cavity.

Photorefractive oscillators using inorganic PR crystals have been studied for
some time. The first announcement of a resonant cavity using a PR polymer has
just occurred in the past few weeks (March, 1998).

(From: Douglas Dwyer (ddwyer@ddwyer.demon.co.uk).)

If you are trying to make a laser gyro as a home project you've got a lifetime
project.

I think the ring laser is often carved out of a solid block for stability , a
major problem with both ring lasers and fibre gyros is locking of the two
phases - when rotated the phase relationship between the two paths sticks
until a certain rotation rate is reached at which point the two paths unlock
and it starts to work properly The solution to this could be to deliberately
modulate the phase of the light with pseudo random noise and demodulate at the
phase detector. Also as stated the fibre gyro is less attractive because of
the inherent greater spectral width of the laser.

I wonder if one could bake a Mossbau gyro. I once saw turntable rotation
detected by the relativistic effects on the gamma radiation and absorption.
That could be easier.

Fourier Optics

The Fourier Transform (FT) of a signal - be it one dimensional such as audio
or RF, or multidimensional such as an image (picture) - is a powerful tool for
the analysis and processing of information. In a nutshell, the FT provides
information on the frequency content of the signal. The signal and its FT
form what are known as a 'transform pair'. The FT is a completely reversible
operation so if the FT of the signal is completely known, the signal is also
completely determined.

Some applications for the Fourier transforms include:

Analysis of signals to look for features such as a particular set of
frequencies. By simple pattern matching of frequency amplitudes and/or
phases, recognition of voice or image content is possible.

Implement filtering operations such a high pass, band pass, low pass,
matched filtering, etc. Convolution in the signal domain is equivalent to
multiplication in the frequency domain so once a filter (convolution) kernal
exceeds a certain size, transforming into the frequency domain, multiplying,
and transforming back is more efficient than performing a large convolution
directly.

Refer to any book on signal processing for more details Fourier analysis and
applications including all of the exciting equations!

The usual modern way of performing the Fourier transformer operation is to
digitize the data and use a special optimized computer algorithm called the
'Fast Fourier Transform' or FFT. However, even the most efficient variation
of this approach is highly computationally intensive - especially when large
multidimensional arrays like high resolution images are involved. To achieve
adequate performance, digital signal processing accelerator cards,
multiprocessors, or even supercomputers may be needed!

Enter Fourier optics.

It turns out that under certain conditions, a simple convex lens will perform
the Fourier transform operation on a two dimensional (2D) image totally in
*real time*. The theoretical implications of this statement are profound
since real-time here means literally at the speed of light. In practice, it
takes great effort and expense to make it work well. Many factors can degrade
the contrast, resolution, and signal-to-noise ratio. Extremely high quality
and expensive optics, precision positioning, and immaculate cleanliness are
generally essential to produce a useful system. However, to demonstrate the
basic principles of Fourier optics, all that is required is a common HeNe
laser and some relatively simple low cost optics.

You don't even want to think about what a high quality Fourier optics setup
for serious research would cost. However, for demonstrating the fundamental
principles, it is possible to get away with much less. The necessary
components are shown below:

A laser with a long coherence length is required. A diode laser will probably
not work well. Therefore, this is likely to be a HeNe type. A medium
power laser (i.e., 10 mW) will make for a brighter display but a 1 mW should
work just fine. CAUTION: Take appropriate precautions especially with a
higher power laser. However, once the beam has been collimated to a large
diameter, the hazards are reduced.

Ideally, you have a nice optical bench to mount all these components.
Otherwise, you will have to improvise. The first three items (the spatial
filter components) really do need to be accurately and stably positioned. See
the section: Laser Beam Cleanup - the Spatial
Filter.

Laboratory quality lenses for Fourier optics research cost thousands of
dollars each. However, you can demonstrate the basic principles and do
some very interesting experiments with inexpensive optics.

The first three components constitute the 'spatial filter'. This is
needed to clean up the raw laser beam to eliminate off=axis rings, ghosts, and
other optical noise. See the section: Laser
Beam Cleanup - the Spatial Filter.

FL - Focusing Lens is a short focal length convex lens. It can be a simple
lens, microscope objective, microscope or telescope eyepiece, etc. The
focal length should be quite short - at most 1/4 inch - since you will want
to be able to expand the beam to an inch or two. Or, to ease the setup
requirements for the spatial filter, use a longer focal length lens for this
and an additional beam expander prior to the collimating lens.

SF - Spatial Filter just a pin hole placed at the focus of FL. This is
needed to 'clean up' the laser beam. The smaller the better as long as
you can position the beam's focus to pass through it. This is one place
where a precision X-Y stage is highly desirable since the best pinholes
are only a few um in diameter! (You can make the pinhole easily enough by
using a pin through aluminum foil against a sheet of glass).

CL - Collimating lens is a medium focal length convex lens with focal
length F2 positioned to produce a parallel beam. Its diameter will
determine the size of the transparency, transform place filter, and output
images. Larger is better but will reduce the brightness for viewing of the
transform and output images. I would think that a 1.5 inch diameter lens
should be usable with a small laser (e.g., 1 mW) in subdued light.

The ratio of F1/F2 should be roughly the same as the ratio of the diameters
of the useful aperture of CL (desired diameter of the field of view) to the
HeNe beam.

For example, with a laser producing a 1 mm diameter beam and a useful field
of view diameter of 1 inch, the following will work:

I just finished a class in this, using "Linear Systems, Fourier Transforms,
and Optics", by Gaskill (Wiley).

A coherent source yields a Fourier transform of the electric field, including
the phase factors. An incoherent source will perform essentially the same
effects on the radiance, rather than the field. A coherent source is used to
develop the concepts, and so most of the books show the experimental
verifications of spatial imaging with coherent sources.

A negative lens will give a virtual image. If you want to perform spatial
filtering, I think you're forced to use a positive lens. You also perform
the inverse transform with another positive lens. You should therefore be
able to confirm basic spatial filtering concepts with a hobbyists' telescope.

Gaskill talks about a few special configurations, but the easiest to get to
is to locate a laser to one side of the lens, place the transparency at the
front focal plane, and find the Fourier transform plane at the point where the
point source (a laser) comes to focus. To make things really simple, put the
laser twice the focal distance away from the lens, the image at the focal
distance, and find the FT at twice the focal distance on the far side of the
lens. An alternative is to take a laser, collimate the light to obtain plane
wave illumination, place the image anywhere between the source and the lens,
and find the FT plane at the focal distance on the other side of the lens.
It is the focal point of the light source that determines the position of the
FT plane.

Like I say, I just took the class, am still shell-shocked, and haven't had
a chance to absorb or experiment with these techniques, so I could be
misunderstanding the text.
(From: Norman Axelrod (naxelrod@ix.netcom.com).)
Yes, you need a laser. HeNe works, but not a diode (the laser needs to have
good coherence). Focus the laser through a pinhole (focusing lens and pinhole
combination is called a spatial filter). then re-collimate the light with a
lens. Place the image or aperture 1 focal length from the collimating lens,
then you can either use a bare screen placed at distance away, or a second
collimating lens. This is necessary to get the far-field pattern.

(From: Brian Rich (science@west.net).)

A really cool book about this that I have a copy of but may be out of print is
"Laser Art and Optical Transforms" by T. Kallard. Look for it at a good
university library.

(From: Norman Axelrod (naxelrod@ix.netcom.com).)

There is another way to phrase what is happening that might make it more
intuitive for folks with more of an optics background.

First, the light used should be parallel and coherent.

The light transmitted through the transparency (or light reflected from a
2-dimensional image) is diffracted by the transmission and phase changes
provided by the image. As is done in elementary physics, a lens (here, a high
quality lens) is used to take the light that is diffracted at different angles
and focus them at a distance of one focal length from the lens (just like a
burning lens, except you use parallel coherent light coming into the initial
transparency and you have more than one beam at the burning distance).

The key physical point is that the Fraunhofer diffraction pattern of an object
is the Fourier transform of that object. This is true in the sense that the
amplitude and the phase of the radiation at any point in the diffraction
pattern are the amplitude and phase at the corresponding point in the Fourier
transform.

For simple examples:

The diffraction pattern of a slit is perpendicular to the edges of
the slit, no matte what direction the slit has.

The diffraction pattern of a small circular object has circular symmetry.
The intensity appears to have circular rings of light with intensity
variations as you move out from the optical axis.

Arrays of identical apertures provide diffraction patterns that are the
product of the intensity patterns from the individual apertures and the
intensity patterns from the geometry of the array. If the array is random,
you get the diffraction pattern of the individual apertures. Young (of
Young's Double Slit) used this for one of the earlier measurements on the
diameter of blood cells.
One of the more amazing things (at least to me) that you can do with this is
to take remove the horizontal OR vertical lines from an image of a wire screen
with crossing vertical AND horizontal lines. By a simple modification of the
light in the transform or diffraction pattern plane, you can produce an image
that ONLY has either vertical or horizontal lines!
The Fourier transform or diffraction pattern from a wire screen (like a screen
from nylon stockings or from on a screen door - - but with tighter geometry)
with periodic holes on a square grid consists of bright regions on a similar
square grid. If you take an opaque screen and put a long narrow opening to
allow ONLY the light from near the x-axis to get through, the resulting image
has only vertical lines! This is called the Abbe-Porter experiment (and is
discussed in Goodman's book).
We have patents on this in which we used a simple opaque cross (in the
transform plane) to eliminate perpendicular lines in an image and re-image
only non-rectangular features. The perpendicular lines (lined up with the two
axes of the cross) are effectively eliminated, but circular features and
irregular features are imaged just fine!
My favorite book on this remains Optical Physics by Lipson & Lipson (Cambridge
Univ Press).

(From: Tom Sutherland (tom.sutherland@msfc.nasa.gov).)

Please allow me to recommend Professor Goodman's excellent and recently
updated text "Fourier Optics". If I had my (last edition) copy in front of
me I'd give you a better answer, however I do recall that the exact fourier
transform of a pattern illuminated by a coherent plane wave is produced at
the back focal plane of a lens if the pattern is located at the front focal
plane of the lens. The intensity (but not the phase) of the fourier
transform is produced if the pattern is located anywhere else in front of
the lens (but of course there are some questions of scaling).
(From: Robert Alcock (robert@fs4.ph.man.ac.uk).)
Have a look at the book "Introduction to Fourier Optics" by J.W. Goodman.
McGraw-Hill Book Company 1968.
The first few chapters set the theoretical framework for the book by
explaining 1D and 2D fourier transforms and scalar diffraction theory. I think
that the chapters that you may find particularly interesting are:

Chapter 5 - Fourier transforming and imaging properties of lenses

Chapter 6 - Frequency analysis of optical imaging systems

Chapter 7 - Spatial filtering and optical information processing

It's a fantastic book that should answer all your questions.

(From: Herman de Jong (h.m.m.dejong@phys.tue.nl).)

Let me explain the optical Fourier Transform by lenses with an example: Suppose
for simplification we essentially look at a two dimensional system: we use
cylinder lenses and slit object.

When you use a broad laser beam and eliminate a slit (a pulse function), it
will have a near field and a far-field pattern that is not exactly the
same. The far-field pattern is a utopia but you get very close to the utopia
the further away you put your screen. The intensity pattern is a squared sinc
function (the sinc function is the FT of the pulse function) that scales with
distance. We conclude the infinity pattern to be the squared of the FT of the
slit and the associated E -field is actually the FT. If you use a cylindrical
lens to image the slit on a screen you also get an FT provided you collect all
relevant light from the slit onto your lens and the lens is perfect. It scales
with the ratio of object an image distances It so happens that the FT of the FT
the original but for a scaling factor and a minus sign in the inverse FT. I'm
not sure how but in otical intensity FT's it makes no difference probably
because of the squared of E -field that eliminates the minus sign.

It gets much more difficult to grasp with 3D and rotationally symmetrical
optics, objects and images. You wouldn't want to know and I wouldn't be able
to answer many questions.

(From: James A. Carter III (carter@photon-sys.com).)

It is possible to form the Fourier transform by placing the transparency in a
convergent-cone optical field formed by a single laser. This technique is
used when one wishes to scale the transform to be optimally sampled by a
detector with fixed spatial sampling. Changing the location of the
transparency with respect to the focus of the cone (i.e., changing the
quadratic phase of the optical filed) will change the scale of the transforms
as it maps spatial frequency (sometimes called the "plane wave spectrum") to
spatial coordinates. Actually, no lens is required at all if you have a large
enough lab and can invoke the "far field" condition. The "Fraunhofer"
condition uses the quadratic phase of the lens to negate the second order term
in the scalar diffraction integral using denoted as "Fresnel" diffraction.
The far field condition puts the observation plane far enough away from the
transparency plane to make it essentially a constant term in the integral and
again you have a 2-D Fourier transform.

The lens can be thought of as a way to image the far field (ideally at
infinity) to the back focal plane. If the transparency is not at the front
focal plane, then the transform field (amplitude and phase) at the focal plane
will have a quadratic phase term. The quadratic phase is irrelevant if the
field is detected (with detector or film) because then all phase information
is lost. If the field is recorded with a reference phase (i.e., a hologram),
or is filtered for subsequently inversing the transform, then the quadratic
phase should be corrected. The simplistic way to do this is to use a plane
wave illumination (collimated source) and place the transparency at the front
focal plane. Using your imagination and knowing the symmetry of the Fourier
transform should justify this rational.

The field at the transform plane contains only the information that is
collected and sampled by the lens. Thus, the ability to sample higher spatial
frequencies depends on the collection angle (numerical aperture) of the lens.
Some feel that the illumination beam must be spatially filtered to produce a
uniform distribution. This is no more the case than saying that every Fast
Fourier Transform should just be zero padded. Hamming, Hanning and other
windowing algorithms are used to suppress the side-lobes produced by the
finite sample extent. The Gaussian distribution of the laser can actually
improve the fidelity of the transform and eliminate "ringing." The quality of
the lens in terms of wavefront aberrations is important, but no more important
than the wavefront quality of the beam. These phase aberrations may effect
the point spread function of the system (seen when no transparency is present)
and it is the point spread function that convolves with the transform and
limits fidelity.

The text by Jack Gaskill and Joe Goodman are excellent for details. Another
excellent source is the "(The New) Physical Optics Notebook: Tutorials in
Fourier Optics" by Reynolds, DeVelis, Parrent, Thompson. This is available
from Optical Engineering Press (SPIE). The "old" version of this was used in
my training at the U. of Rochester when I took physical optics from one its
early authors (Brian J. Thompson).

Many interesting things can be done with this simple engine.

(From: Jeff Hunt (jhunt@ix.netcom.com).)

I'm a grad student at the Optical Sciences Center at the University of
Arizona, and I think that Jack Gaskill's book on the subject is quite good.
Just like Gaskill says, it covers what Goodman's text does, but it explains
things in a way that is easier to understand (Goodman is the authority on the
subject, from what I understand.)

(From: DeVon Griffin (devon@baggins.lerc.nasa.gov).)

Having done Gaskill ten years ago, I would say that the main drawback of the
book is his notation. The m double-hat triple prime sort of thing makes
trying to pick it back up after not having looked at it for awhile a daunting
task.

Barcode (UPC) Scanners

The use of the Universal Product Code (UPC) has revolutionized
grocery/supermarket and other retail store checkout and inventory control
as well as being applied to other numerous and varied applications including
package routing and tracking, and even tagging of wild animals and an aborted
attempt to use similar codes printed in your weekly TV section to program your
VCR with a hand-held barcode wand!

Some would argue that the use of such technology in supermarkets at least, has
dehumanized the buying experience and stacked the deck in favor of the
merchant since prices tend to no longer be printed on each item and the
checkout process is now so fast that it is virtually impossible to catch
mistakes should they occur. Since the price-to-item relationship is stored in
a computer somewhere, it is indeed possible for there to be errors - but in
reality, these are generally rare.

Space and other factors prevent me from going into the details of the
Universal Product Code itself but here are some Web sites that have info
and many links to barcode manufacturers, barcode specifications, barcode
generating software, and other information that may be useful:

The quick summary is that the pattern of black lines familiar on virtually
all products nowadays - the UPC code - has been carefully designed to be
easily decoded when scanned in either direction, at an arbitrary angle, and
with variable speed. There are actually many other barcodes besides the UPC,
used for inventory control, tracking, and other diverse applications. (If
you should need to stay in a hospital, you will be given a barcode!)

The UPC consists of 12 total digits. The first digit is the type of product
(0 is for groceries, 3 is for drugs, etc.), the next 5 digits on the left
half are the manufacturer code, the first 5 digits of the right half are the
product code, and the last one is a modulo check digit. Each digit as its
name implies can have a value from 0 to 9, encoded as a set of 4 alternating
bars and spaces, each of which may have a width of 1, 2, 3, or 4 units called
"modules". The total width of each digit is defined to be 7 which allows for
20 unique codes - 10 used for the left 6 digits the other 10 for the right 6
digits. The left six digits are coded with odd parity; the right six digits
with even parity. Additional details can be found at the first Web site,
above.

For the purposes of the discussion below, we restrict our attention to the
type of equipment found at your local supermarket - the barcode scanner that
is mounted under or beside the conveyer counter (and may include an electronic
scale but that is another story). While details vary, the basic architecture
of these devices tend to be very similar. Once you are familiar with one
model, parts identification and the optical path of any other one will be
almost immediately obvious. Hand-held scanners may not even use a laser but a
linear array of LEDs. Large industrial barcode scanners may contain a much
more powerful laser and somewhat different optical path. Some of the newest
barcode technology does away with the laser scanner altogether and uses a 2-D
video camera (CMOS or CCD) based imaging system and high speed DSP (Digital
Signal Processor) instead. This eliminates most of the complex and costly
optical and mechanical components making for a compact robust system. But
currently, the traditional electro-mechanical laser scanner is still most
common.

The basic principle is to use a collimated laser beam, rotating multifaceted
mirror, several stationary mirrors, and other optics, to generate a scan
pattern above or beside the scanner which will intercept the UPC code printed
on the item to be scanned in almost any orientation. While the scan may
appear to consist of multiple lines or a continuous pattern, it is in reality
a single rapidly moving spot.

Looking through the glass of the scanner, it may appear that all sorts of
stuff is arranged at random. However, this is not the case. :) Refer to
Optical Path of Typical Checkout Barcode Scanner
as you read the description below (which also includes some comments on
potentially useful parts that may be obtained from these units):

Laser - The source of the beam is either a low power helium-neon (HeNe)
or diode laser. Older (and larger) scanners tended to use HeNe lasers.
However, size alone is no sure indication until you get to very small (6 inch
cubes or hand-held wands) which are almost always based on diode lasers (if
they use a laser at all). A better test is to check the color of the beam -
the light from HeNe laser based scanners appears orange-red (632.8 nm) while
that from diode laser based scanners tends to be a deep red from the 670 nm
wavelength which is less expensive (but just as effective). Just explain
that you are doing scientific research when the people in the white coats
come to take you away for staring into the scanner! :)

HeNe lasers are typically 1 to 3 mW (mostly near 1 mW) using tubes
between 5 and 10 inches in length. The divergence of the raw beam from HeNe
laser tubes use in barcode scanners tends to be greater than the diffraction
limit (possibly 5 to 10 mR or more), probably to save the cost of an extra
beam expander. Some HeNe tubes even have a lens glued to the output mirror
to mess up the beam in this way! (However, the beam quality is just as good
as a similar 1 or 2 mR divergence tube - it just looks more like a
flashlight than a laser!) Where an external lens is actually glued to the
output mirror of the HeNe tube, it can probably be removed with a suitable
solvent or some heat leaving a low divergence tube. See the section:
Disassembling Cemented Optics for more
details. Or, locate the collimating lens that is present in the scanner or
one of your own, and use that to adjust the divergence or focus of the beam
as desired. In fact, since you can intercept the beam wherever you want,
using the high divergence tube provides added flexibility to generate a very
low divergence beam (e.g., 0.1 mR) with a single additional lens.

The HeNe laser power supply may be a self-contained 'brick' or built onto
the mainboard. The former is of course much more desirable from the
perspective of salvaging parts! In either case, to turn on the laser will
probably require grounding or pulling up an enable signal since in most
systems, the laser is automatically turned off after a period of inactivity.

Diode lasers are typically 670 nm (deep red) with 5 mW maximum output.
A collimating lens and possibly some other optics will be part of the diode
laser assembly.

The laser diode driver circuit will be in close proximity to the laser diode
itself and may be on a separate board. However, it is most likely part of
the mainboard. and difficult to determine correct use without a schematic.

Variable attenuator - A graded density filter may be present immediately
following the laser's output to adjust the beam intensity to compensate for
variations in laser power (mostly for HeNe lasers - diode lasers will have a
pot for this purpose).

Turning mirror(s) - There may be one or more high quality planar first
surface protected aluminum or enhanced aluminum (or similar) or dielectric
mirrors to direct the beam. (If dielectric, the peak reflectivity wavelength
will match that of the laser used possibly also optimized for the actual
reflection angle in the scanner.) Their mount will probably be adjustable
in X and Y to some extent. While no where near the precision of a Newport
MM2 mirror mount, they should be perfectly adequate for basic optics
experimentation when attached to a rigid surface. Note that these mirrors
do not serve any fundamental purpose - they simply fold the optical path
resulting in a smaller physical package.

These mirrors - particularly the dielectric type - are often of high enough
quality to be used inside a laser resonator - even that of a low gain
type like a HeNe laser. Almost every barcode scanner dielectric mirror
I've tried would result in lasing when used as the external mirror with
a one-Brewster HeNe laser tube. The intensity and beam quality generally
weren't quite as good as with a proper laser mirror, but they did work.
Depending on the type of coating and the finish of the other surface,
these may produce a useful output beam. About half the aluminum mirrors
tried in tnis manner also worked, but with much lower performance.

Main objective combo - The outgoing and return beams (reflection from
the item being scanned) follow the same path except that the return is not a
nice narrow collimated beam. The outgoing beam passes through a window
inset into a large positive lens. This may include a mirror or prism to
redirect the beam and the collimating/focusing lens to produce a nice small
spot at the item being scanned. This is a strange part! The large lens
focuses the return beam onto the photodetector (see below) and since optical
quality isn't critical at this point, it is likely made from molded plastic.

The components of the this part can generally be separated to use individually
using a combination of brute force and solvents. For example, to remove the
lens and prism from the combo in the Orien 300, a pad of tissue paper is
inserted in the hole followed by a wooden dowel that just fits. A couple of
whacks to the dowel with a small hammer while holding the assembly should
result in the prism/lens popping free. They can then be separated by soaking
in acetone (nail polish remover). WARNING: Acetone and its vapors are
flammable and toxic. CAUTION: Acetone will also damage many plastics
including most likely, the large plastic lens, so don't let it contact that
or other plastic optical components.

Multifaceted rotating mirror - The collimated outgoing is deflected by a 3
to 6 facet polygonal mirror directly driven by a speed regulated brushless DC
motor. The motor/scanner assembly is generally a separate module in older
equipment requiring only DC power and an enable signal to run. However,
like the HeNe power supply, newer systems usually mount it directly on the
mainboard. Most of the larger barcode scanners use motors of similar quality
to those in disk drives since they may need to run continuously. However,
smaller scanners may use something that looks like the core of a cheap DC fan!

Unlike those in a laser printer, the mirror facets are large since they have
to reflect the diffuse return beam as well as the tiny spot of the outgoing
beam. They are fabricated as individual mirrors glued to a cast metal
wheel type affair and are all set at slightly different angles so that each
rotation of the mirror wheel results in scan lines at 3 to 6 slightly
different locations depending on the number of facets.

Multiple planar mirrors - The final optical component before the outgoing
beam hits the item being scanner are several large fixed first surface
mirrors. Despite their appearance of having been just plunked down at
random, these have positions and orientations carefully chosen to direct the
beam in a pattern providing the best chance of intercepting the UPC code
with at least one scan line, and to result in approximately equal spot
scanning speed for each scan line. Depending on design, the beam may strike
either 1 or 2 of these mirrors on its journey each way (out and back).

These are usually decent quality aluminized first surface mirrors and could
find all sorts of other uses. Although generally shaped as strange 4 sided
polygons, they can be subdivided into more useful sizes using a glass cutter
from the rear or a water-cooled diamond cutoff wheel.

And for those who fear lasers of all types, there is absolutely NO risk to
vision or anything else in looking into the business end of a barcode scanner.
The laser is low power and the beam is moving, so it's unlikely you'll even
experience any afterimages. Staring into the scanner all day would cause
no harm except possibly cramps from being stuck in one position for too
long. ;-)

The outgoing beam is set up to be a small spot in the active area above or
beside the scanner - the scanned item volume. However, the return from
the UPC printed on the item is in general not well focused but is a diffuse
reflection. Thus, as noted, all the mirrors have to be large to capture as
much of this as possible to feed to the photodetector. The return path is
the same as the outgoing path until the objective combo lens. This focuses
the return beam onto the photodetector:

Photodetector - A silicon photodiode, often of moderate area (typically
2x2 mm, good for a laser power meter) intercepts the return beam focused by
the objective combo. (Some light is lost to the inset optic for the outgoing
beam but this is small.) There may be an additional focusing lens and/or red
ambient light blocking filter associated with the photodetector.

Controller - A microprocessor based system analyzes the datastream from
the photodetector, isolates any section that appears to be valid data,
decodes the UPC symbols, and sends the result via an RS232 line (or possibly
a proprietary interface for older models) to the host computer. Handshaking
assures (hopefully from the point of view of the store owner!) that data
isn't lost.

Power supply - Depending on the model, these may plug directly into the
AC line or be powered from a wall adapter.

See the document: Sam's
Gadget FAQ for more on salvaging parts from barcode scanners.

There really aren't too many safety issues with respect to these devices
even though they contain a Class IIIa (1 to 3 mW) laser and the beam may appear
to be quite bright. (Note that barcode scanners systems are listed as Class II
laser devices since access to the laser and optics requires some disassembly.)

There is really no risk to the user or customer in proximity to a checkout
scanner. The laser beam is moving rapidly and is low power. A rough
estimate of the maximum possible eye exposure to a properly functioning
scanner is about 10 microwatts or less. The only possible risk would be
if the scanner motor failed for some reason and the laser beam was stationary.
However, (1) some if not all scanners have a safety device to shut off the
laser should the return beam not behave properly and (2) with laser power of
around 1 mW, the normal blink and aversion reflex should provide adequate
protection. The perceived brightness is somewhat of an illusion due to the
peak intensity and pulsed nature of the beam.

When poking around inside a barcode scanner, there are somewhat greater
risks of being dazzled because the laser beam will be stationary and
collimated along portions of the optical path. However, more than very
temporary after images are unlikely.

For AC line powered units (no wall adapter), there will be some exposed
115 or 230 VAC points near the line cord and on the mainboard or power
supply. For HeNe laser based systems with the high voltage power supply on
the mainboard, there will be exposed pads with voltages up to 5 kV or more
(during starting). Since these may not be clearly marked, it pays to
identify them beforehand and take appropriate precautions. Those with 'brick'
type HeNe power supplies are usually pretty well insulated.

Then, there is the rotating mirror which can catch long hair or jewelry.

Finally, since these scanners may have seen service under less than sterile
conditions with all sorts of icky and disgusting stuff passing their way
including meat and chicken parts dripping with blood, there can be all sorts
of surprises in store for you from mummified mice to maggot colonies. Take
appropriate precautions in your exploration and/or disassembly!

The unit I have which uses a power supply 100 percent identical to
the schematic and PCB layout of IC-HI1 is a Metrologic Model MH290. It is
labeled with a 1990 date of manufacture and says 12 VDC at 550 mA on the
scanner unit itself. The wall wart that runs the system is rated at 12
VDC at 1 A.

The MH290 is a hand-held unit with a trigger, you pull the trigger when you
are ready to scan and the laser starts scanning for 4 or 5 seconds and then
shuts down. To attempt a second scan, you have to pull the trigger again.
Inside the hand unit there is the receiver, a second PCB to support the
receive electronics and the spinning mirrors (driven by a small 15 degree
per step stepper motor). The MH290 is smart enough to know when the laser is
on, and the error is produced if it doesn't come on OR if it stays on longer
than it should.

The MH290 connects to another unit via a 9 pin RS232 type connector, the
other unit has the EEPROM and related components for decoding and
interfacing to the computer itself. The MH290 hand held scanner does not
connect directly to the computer and all power sent to the MH290 comes from
this other box.

CD, DVD, and Other Optical Disc/k Systems

There are two consumer oriented applications for lasers that are by far
dominant, at least in terms of the number of units produced. These would be
laser printers and related equipment, and CD, DVD, and Blu-ray and
HD DVD players and drives. Laser printers aren't really very interesting
from a technology perspective - an IR laser diode with fancy focusing
and scanning optics. (I suppose laser pointers are also something
that should be included as being a common consumer laser but it's not clear
how many of these are actually used for their intended purpose!)

Optical storage, of which CD, DVD, MiniDisc, and LaserDisc are a subset,
are all based on very similar technology requiring extreme precision to be
able to read (and perhaps write) micro-size features on a spinning disc/k.
The first optical drives were developed in the 1970s using HeNe lasers.
LaserDisc players were the first consumer electronics to benefit. Early ones
used HeNe lasers but even more modern LaserDisc players seemed to simply
substitute an IR laser diode for the HeNe laser while retaining much of
the other optics without significant miniaturization. However, LaserDisc
players were never the same sort of mass produced product as the
CD player and were more directed to high-end and specialized markets
like interactive education and training since the disc format allowed
rapid access to video snippets or up to 54,000 individual video frames
on a LaserDisc. With the introduction of the personal computer around
the same time, the LaserDisc was an ideal video storage peripheral,
unsurpassed until the advent of the DVD. (And some people would claim
still superior.)

For more information on optical storage technology, see the
Notes on the Troubleshooting
of Compact Disc Players and CDROM Drives. In addition to descriptions
of how the technology works, there are photos and diagrams of optical
pickups ranging from one in an HeNe laser-based LaserDisc player prototype
through modern DVD drives. The size difference is dramatic with the typical
DVD pickup being roughly 1/1000th the volume of that LaserDisc pickup. Yet,
it must perform all the same functions.

Laser Printers and Similar Equipment

All modern laser printers use IR diode lasers of 5 to 30 mW maximum output.
Their wavelength is generally around 780 nm (like those of CD and many other
optical disc/k systems).

Very old laser printers used helium-neon lasers but these are even rarer than
HeNe laser based LaserDisc players. However, if you do find one, there will
likely also be an Acousto-Optic Modulator (AOM) and driver since directly
controlling HeNe lasers at high speed isn't feasible - don't neglect these
very desirable components!

And, of course, those large graphic arts machines may have large HeNe lasers
and even air-cooled argon ion lasers though newer ones will use Diode Pumped
Solid State Frequency Doubled (DPSSFD) green lasers.

The optical path from laser to photosensitive drum is in the order listed
below:

Laser - Usually semiconductor laser diode mounted in assembly with
collimating lens and other optics. Its output is a nearly parallel beam
1 or 2 mm in diameter.

Laser diode driver - Usually a circuit board in close proximity to the
laser diode which provides power and modulation capability. Reverse
engineering and luck may be required to figure out how to use it. A
thermo-electric (Peltier) cooler may be present on some models.,

Multifaceted scanner - A polygonal mirror (usually metal coated) on a
high quality brushless DC motor. Unlike barcode scanners, the mirror in
this case is very thin (height-wise) since it only has to intersect the
fixed outgoing beam. The constant speed controller may be a part of the
motor assembly with only a few interface signals to power and run it.

Objective lenses - 2 or 3 lenses that look cylindrical may actually be
just relatively thin sections of normal lenses or anamorphic with different
focal lengths in the two axis. Due to their shape, they may be of
questionable utility for other purposes.

Mirrors - 1 or 2 long strip mirrors that may be metal coated (with a
copper tinge for IR laser printers) or dielectric coated. Note that while
these appear to be planar, they may in fact have a slight curvature along the
narrow axis. The dielectric types may be of very high (laser resonator)
quality but if frosted on the opposite surface, of limited utility since any
transmitted beam is dispersed.

Fiber optic sensor - Canon engines have this situated at the one end of the
scan line to detect the beam and provide a time reference.

The laser and optics components in laser Fax machines are similar but in
addition, there will be the cold cathode fluorescent lamp, imaging lens, and
CCD array of the input section. In principle, this could also be a laser
scanner with virtually identical construction to that of the printer but I
don't know if this is ever done in practice.

Laser Light Shows Lasers

For more information on lasers suitable for light show and related multimedia
entertainment applications, see the Chapter:
Argon/Krypton Ion Lasers. For more
information on all aspects of laser light shows, check out the
LaserFX.com Web site.

(Portions from: Erik Huber (erik.p.huber@uibk.ac.at).)

I worked in a big disco as LJ - Did a lot of raves and such stuff. I also DJ
a little just for fun. The laser power you need depends on the room you have.
If you want to scan pictures you need more power. If you just use rays, you
won't need so much.

Laser Diodes 635 nm (bright red) 10 mW, about $150 to $250. Use these for
beams in a smoked room and it will look cool.

WARNING: Be aware that the maximum laser power level for the human eye is
about (2.5 mW)/(cm2). Never look into the beam!

(From: Steve Roberts.)

If you wish to scan graphics on clouds, it takes from 10 to 20 watts of well
collimated argon light to do so, and the problem is only people within about a
10 degree cone around the laser site from where it hits the cloud will see the
graphics. Everybody else at best sees a faint flash from within the cloud, and
in most places in the US the conditions for doing it will only be right a few
days a year. It's also not a good surface for images, any thing more then a
simple logo or spirograph pattern is unlikely to be recognizable. Scrolling
text didn't work. How do I know, I was the one running the spirograph
generator as a guest at the laser site.

In the USA, laser shows in clubs/bars/parks are regulated by the CDRH (Center
for Devices and Radiological Health, a division of the FDA). Audience
scanning is NOT permitted in the USA while it is common in the rest of the
world. A large scanned effect spreads the laser power over a wide area and
usually has some motion to it (such as the sine waves used to make rippling
sheets of light). This means that the energy density and the exposure times
are low.

If the laser beams are not scanning directly on the audience (dancers) then
the effects are probably safe. If the system uses scanned beam effects, then
it is probably following the rules of it's jurisdiction and is probably safe.

Having done a few of those shows overseas, it's not just moving fast, but
that's part of it. In fact, moving too fast can in some cases brighten the
beam to exceed the MPE (Maximum Permissable Exposure) because of the dynamic
characteristics of the scanners. It's the dwell time on each point of the
image as the scanners are tracing it out, it has to be carefully measured for
each animation or effect with a scope, fast photodiode, and a laser power
meter. Each image has to be carefully designed using the show software to
avoid sharp corners and other hotspots. Just scanning it fast is not enough -
you will note that only very large scans flow over the audience. There is
what is referred to as the zero line, well above the audiences head. As the
images dip below the zero line, they are reduced in brightness by the hardware
and by the show programmer. A scan fail system is also usually in use that
will cut off the system should a scanner fail, and this has to happen fast if
the MPE is not to be exceeded, in a fraction of a millisecond, so very careful
engineering has to go in this.

Please folks, just because you saw a beam scanned over the audience in a club
and you have a laser, don't try it at home without getting the equipment to
make the measurements and calculate the MPEs. It is not possible to determine
if a effect is eye-safe by eyeball alone. The European clubs pay between
$50,000 and $100,000 for these systems so a lot of time and money is spent on
doing the safety analysis when programming a show. There are permits and
licenses involved as well. Each frame of the show - and there is usually
6 to 15 frames per second - must be checked and carefully designed when doing
this. The show must be checked for each facility it is ran in as well. You
really need to take a class in how to do it safely. Such classes are offered
at ILDA and ELA meetings and by safety inspectors/laser providers in Europe.

Please note that this is not normally legal in the US as we have lower MPEs
that make it ineffective when done anyways. It was suspended in much of Europe
recently for a review of the power levels in use, new standards were
implemented with tighter controls and it is again legal in parts of Europe. It
is also legal in Canada, but again, measurements have to be made.

If you're gonna do a show and you don't know what your doing, the basic
guidelines for where the beam may go are a minimum of 3 meters up from the
highest point in the audience and a two meter horizontal separation from the
audience to any beam. In the USA, a CDRH Variance is required for any public
show above 4.95 mW, and the penalties are draconian for failure to obtain them.
The MPE in the US is about 2.3 mW per square centimeter per second for visible
lasers.

For multiple color presentations, it's possible to use either a single
laser that produces more than one wavelength, or two or more lasers combined
into a single beam using dichroic mirrors. The only commercially available
multiple wavelength solution is to use a 'white light' mixed-gas argon/krypton
ion laser. The advantage is that it's possible to get a reasonable mix of
colors and relative intensities, and the beams are already aligned to
each-other. The disadvantages include large power requirements, high cost,
and possibly relatively short life. The wavelength balance also changes
with age, use, and output power. However, there is really is no viable
alternative at present to obtain suitable red, green, and blue wavelengths
from a single laser. Solid state RGB lasers are never built as single lasers,
but either multiple lasers, or one or more lasers with some other wavelength
conversion scheme added on. These are very complex and expensive but
will no doubt improve and come down in price as demand increases.

Where a non-gas laser solution is desired, what is often done is to
combine green (532 nm) and blue (473 nm) DPSS lasers with a red (635 or
650 nm) diode laser, though red DPSS lasers (656.5, 660, or 671 nm) are
now becoming available. However, this is 3 separate lasers whose beams
need to be matched in size and divergence, and possibly polarization, and
then combined into one.

(From: Wes.)

For our 1.4 to 1.5 Watt RGB system, we use the following:

700 mW of 650 nm red (multimode diode).

350 mW of 532 nm green (frequency doubled DPSS Nd:YVO4 at 1,064 nm).

350 mW of 473 nm blue (frequency doubled DPSS Nd:YAG at 946 nm).

This combination makes a nice white balance. You could use 50% less red if
you have 635 nm red. If you have 671 nm from a red DPSS laser, might as well
forget it (would require way too much power since very very low visibility
of that wavelength). FWIW, I think 650 nm makes better colors than 635 nm.

(From: John R. (scifind@indy.net).)

In my opinion, I would rather have a single mixed-gas 'white light' laser to
avoid the hassles of beam collimation of two independent lasers. This is
especially true if you do shows on the road where everything is jostled
around. You may get better life with a red-only krypton tube, but you are
almost always fiddling with near- and far-field collimation to keep your PCAOM
output efficient across the entire spectrum.

The color balance in a single mixed-gas laser will slowly change over time,
but it is easy to make software color palette corrections on the white-light
balance in a few minutes. (At least until the red output drops too much.)

As for tube lifetime, I think it is function of art, science, tube current,
luck, and the phase of the moon when the tube was installed. I know one
laserist who only got 600 hours on a tube. I know another that has lasted for
many years.

This laser has been reported on in various laser trade publications and
discussed on the USENET newsgroup alt.lasers. Such systems represent the
future direction of technology for RGB laser show and laser TV equipment
due to their higher efficiency and more robust construction. Cost is still
a problem though. :)

(From: Patrick Murphy (pmurph5@attglobal.net).)

The Schneider solid-state RGB laser does exist and is in use for laser
shows, including the Hershey Park outdoor show in the U.S. There are two
main versions of the laser. One is just the laser for light-show type
applications. The other is the laser plus a video projection head (scanning
mirror type) to create infinite-focus, wide color-gamut video. I saw both
versions doing a combined show (video + laser graphics + laser beams), a few
weeks ago at the Schneider factory in Germany.

The following information relates just to the light-show model,
imaginatively called "Showlaser" .

The original $160,000 price mentioned elsewhere was an estimate; the
actual U.S. price will be somewhat lower than this ($120K? $140K?). This is
still a lot, but not quite as much as the estimate. Schneider realizes the
price is high for the laser light show market and will be seeing if it is
possible to lower it.

The useful output power is 13 watts of modulated white-light from the end of
a fiber (e.g., into your scanners). The colors are nicely spread -- red at
628 nm, green at 532 nm, blue at 446 nm -- so you get very dramatic violet
and purple. (In video applications, there is no speckle, skin colors are
normal, and saturated colors are quite striking.)

The input power is 220 VAC at 3,000 W (e.g., about the same as two hair
dryers). It has its own internal chiller, which you fill every few months
with a gallon of distilled water. So in this sense it is "air-cooled", as
you don't have to hook up an external chiller.

Because everything -- laser head, modulators, chiller, power supply -- is
built into one unit, the Showlaser weighs 660 pounds. This is roughly the
same as all the parts of a medium- or large-frame ion laser together. The
unit is compact and is on casters so the weight is not quite as bad as it
could be.

The working part of the laser is manufactured by Jenoptik (it says so it in
a big decal on the Showlaser's side). The working principle is described in
this paper:
RGB
Lasers for Laser Projection Displays. Here is the abstract:

"JENOPTIK Laser, Optik, Systeme GmbH has developed the first industrial
all-solid-state Red-Green-Blue laser system for large image projection
systems. Compact in design (0.75 m 3 , 180 kg, 3 kW power
consumption), the system consists of a modelocked oscillator amplifier
subsystem with 7 ps pulse duration and 85 MHz pulse repetition
frequency, an optical parametric oscillator (OPO), and several
non-linear stages to generate radiation at 628 nm, 532 nm and 446 nm
with an average output power above 18 W. Each of the three colors is
modulated with the video signal in a contrast ratio of 1000:1 and
coupled into a common low order multi mode fiber. The system
architecture relies on efficiently manufacturable components. With the
help of FEM analysis, new engineering design principles and subsequent
climatic and mechanical tests, a length stability below 50 um and an
angle stability below 10 uR have been achieved. The design includes
efficient laser diodes with integrated thermo-electric cooler and a
life time above 10,000 hours. The stability of the output power is
better than +/-2% in a temperature range from 5°C to 40°C. The
system operates reliably for more than 10,000 hours under field
conditions. The design is based (among others) on work by
Laser-Display-Technologie KG and the University of Kaiserslautern."

The working part contains numerous optical components on a breadboard.
Although it looks like a nightmare to align, everything is actually
controlled by a computer. Once it is factory-set, in theory you never need
concern yourself with what is inside. Schneider says the laser will last
10,000 hours before the diodes need replacing.

"AVI-Imagineering With Lasers" is the U.S. distributor. They've received the
one for Hershey Park, with more on order. So far, the Hershey Park laser has
traveled well for AVI. It was trucked five times and four times there were
no problems at all when the laser was turned on. The fifth time there was a
power loss which may or may not have been due to traveling. (The cause is
still being studied.) Since the solid-state laser is much newer than
decades-old ion technology, I think people should expect a few "teething
pains" to be worked out.

Schneider also makes high-end TVs sold in Europe. I have been through the
factory (same place as the laser division) and it is an amazing place, with
raw materials such as plastics and electronic components coming in one end,
and consumer boxed TVs coming out the other. Schneider also recently bought
a majority interest in "tarm", the well-known German laser show company. So
Schneider does things on a big scale, they know what they are doing in
laser, and they want to do it at a consumer level.

Obviously, it's pretty amazing for an RGB laser to get 13 watts of modulated
light from a standard 220 VAC dryer-type outlet, with only occasional water
top-offs, and a 10,000 hour claimed life. On the downside is the weight and
the natural bugs that come with development of any new technology. The price
is the biggest obstacle at this moment. With luck that may be coming down to
a more affordable level, as volume, development, technology etc. improve.

White-light color control with a red HeNe and multiline argon ion laser
and be done without a PCAOM, but you may not like the answer. It is much
cheaper than the PCAOM method, but still involves lots of work and moderate
costs. Of course, if you are a laser hobbyist, nothing is cheap, especially
if you want laser beams other than 632.8 nm red!

For a minimum white light color control system:

You need a multiline argon ion laser with at least the 488 (cyan-blue)
and 514.5 (green) lines.

You will need three separate dichroic filters. (Edmund Scientific and
others sell these).

One dichro is used to split the multiline Argon beam into a transmitted
blue line and a reflected green line at 90 degrees. This gives you the
isolated blue and green beams.

Once you have the separate blue/greens, you need some method of intensity
color. Three possibilities are single-channel AOMs, blanking scanners, or
simple beam shutters.

Once you get the blue/green beams through intensity controllers, they must
be recombined using another dichro.

Using a third dichro, the Argon beams and then super-imposed onto the red
HeNe laser beam. (Of course, you should have some type of intensity
controller for the red HeNe beam as well.)

Thus, the final "white light" beam is made up resultant actions of three
dichros and three intensity controllers. If you have some type of analog
controller for each R/G/B color, you can blend them produce an incredible
amount of colors.

I once built one of these "RGB color boxes" using an argon and HeNe laser. It
worked quite well, but there was the major hassle of alignment of multiple
dichros, other mirrors, and three AOMS. A significant portion of the Argon
power may be lost because it has to pass through three dichros.

As for costs, if you can get surplus AOMs, dichros, and make your own mirror
mounts, maybe $200 to $400 - if you're lucky!

Unfortunately, there is no simple or cheap way of doing it.

And, if you are thinking about mixing yellow and orange HeNe's with argons and
red HeNe's, I seriously doubt you will achieve the performance (and ultimate
cost) of even a used PCAOM.

Why?

Both yellow and orange HeNe's only give a few milliwatts. They will easily
be over-powered by the argon laser not only in terms of actual milliwatts,
but in apparent visual brightness to your eyes.

Unless you are just shining the independent laser beams onto the same spot
on the wall, accurate near- and far-field collimation of a multiline argon
with three yellow, orange, and red HeNe's is almost impossible.

You will need some lots of custom dichros to combine the beams and numerous
beam leveling mirrors to achieve it. Lots of dichros and lots of mirrors
translates into "lots of losses" and a bitch to establish and maintain
collimation. Three dichro color systems are still lots of work. In this
case, you would have a FIVE-color dichro system.

You may also run into problems as each independent laser has its one beam
diameter, divergence, and spatial TEM characteristics. So if you could
collimate them, the resultant "white light" beam will have lots of color
fringes.

Of course, it is your time, money, and effort, therefore, I wish you good
success. But using a higher power red HeNe and then blending it with the
multiline argon is still the better approach.

For more information, try Laser FX.
Their Website author also has an excellent handbook on lasers and laser shows.
There are a couple of chapters devoted to RGB color control in lasers,
including HeNe/Argon methods. If you are serious about making white light
beams (and learning about lasers and shows), this is the book to have!

Also, other ideas. Neos Technology has a 4-channel PCAOM crystal for $680 and
driver for $600. If you are a hobbyist, this is not cheap. However, if you
can get a PCAOM system, it is vastly superior to the RGB/dichro color method.

(From: L. Michael Roberts (NewsMail@LaserFX.com).)

To combine the two lasers your best and lowest cost solution would be a
dichotic. Firstly you need to have a set of two FS mirrors on optics mounts
(E.g., Newport MMI or RMSM OM3/4) to level and steer the beam. Purchase a cyan
or red dichro (from Edmund or PPS); mount it on another optics mount. With a
cyan dichro, you shine the argon through the dichro (which transmits
green/blue wavelengths). Set the dichro in the beam at 45 degrees at the
point where the ar and HeNe beams are made to cross at a right angle.

Careful adjustment of the steering mirror pair on each laser will allow you to
produce two beams that are level relative to each other (and the baseplate of
your projector) and cross at right angles. Set the dichro in the position
where the beams cross at a 45 degree angle relative to the Ar beam (with the
45 degree angle such that the HeNe beam is reflected away from the Ar source).

Adjust the beams until the HeNe and argon beams overlay each other on the
dichro (near field adjustment). Now look at the resultant beam at some
distance or on the projection surface. Adjust the dichro so that the two
spots overlap (far field adjustment).

Adjusting the dichro will cause some change in the position of the Ar and HeNe
beams so you then re-adjust the near field (laser steering mirrors to overlap
the beams on the dichro); then the far field (dichro to overlap beams on the
screen). 2-4 adjustments going back and forth form near to far field may be
required, but in the end you will have the two beams exactly overlaid on each
other. To the eye, the beam will appear a pinkish white - colour balance can
be adjusted by varying the brightness of the Ar laser.

A cyan dichro is recommended as it reflects red and you want to conserve red
photons. You will note that some of the argon beam is deflected in the
direction the HeNe would have been going if not reflected. This is due to
beam splitting at the surface of the dichro. If you use a red dichro, those
would be red photons you would be throwing away.

You can now place a PCAOM (from NEOS or MVM) in the combined beam. Make sure
the polarization of the HeNe is vertical (check the ar while you are at it -
they are usually polarized vertically but poor alignment could have you a bit
off) and that the PCAOM cell is correctly oriented. Varying the control
voltages to the PCAOM will allow you to have additive (RGB) colour control.
You can get 16.7 million colours or more depending on the PCAOM and the system
used to control it.

Dichroic (dielectric) mirrors can be used to split a multiline laser beam into
two or more sets of separate lines. They enable the construction of simpler,
smaller, and more efficient systems compared to dispersive techniques like
prisms or gratings. But good quality dichros are not cheap.

For pricing, you're looking at $20 to $50 a square inch, depending on quality,
and whether a precut size is available. Some may charge a cutting fee or a
little more for the AR coated units. Keep in mind you need to know if you
want CMY or RGB and 0 or 45 degree incidence, as most folks will stock the
whole set of combinations. Be clear - specify that you want "transmit
blue reflect green at normal incidence" Or "pass blue/green combine red at 45
degrees". Most people don't think about it, but "pass deep blue and violet"
for a argon laser turns out to be a nice dichro to have.

Prisms are generally only useful for separating one line, and for laser
display purposes, you need all the power you can get, so you want all the blue
or all the green lines, etc. They are also a pain in the neck as dispersion
versus angle is constant, and a dichro can be tilted off axis quite a bit and
still have throughput. Many traditional laser projectors for planetariums did
just that, have a prism and a color selection galvo, but this takes up several
feet of space to do and is difficult to support from a control systems point
of view and to align. With a prism, you're wasting from 60 to 85% of your
light at any one time, as you're only using one line.

Also beware that Edmund Scientific's dichros are more or less coated for
TV/spotlight applications and thus leak some blue or green that a laser show
dichro wouldn't. This spoils the effect of clean contrasting colors, so you
need a dichro designed for laser display. Edmund's dichros are great with a
tungsten source however.

When you order, ask for backside AR coats on your dichros if available.
Otherwise you'd have 8 to 10% Losses from the Fresnel losses.

The following applies to the visibility of the beam itself (i.e., Star Wars
Light Saber style), not to its appearance then it strikes a surface.

(From: L. Michael Roberts (newsmail@LaserFX.com).)

To create visible beams in *total* darkness you can get away with as little as
100 mW. For beam effects in a club or other venue with some ambient lighting,
1 watt is about the minimum you need to make visible beam effects. Outdoors
you will need 5-6 watts to make visible beams (again depending n ambient
lighting conditions).

In all cases, a scattering medium (smoke or dust) is required to deflect the
light towards the observer's eyes. In clean, clear air in winter, I have seen
the beams from a 20 watt argon look lamer than the beams from a 1 watt indoors
with a good haze.

(From: Steve Roberts.)

In a dark room with average dust levels and high humidity you can start to see
the forward scattering of an HeNe beam at about 1 mW! 30 to 40 mW of argon
makes an OK side view beam in a dim room, but its not exactly a Star Trek
photon torpedo kind of glow. It helps if the argon is configured multiline and
is doing more green then blue, as the eye peaks in the green. To see the beam
in a well lit room requires smoke of some form.

Most laser light show types don't like the common aquafog, it irritates your
lungs after constant exposure, so we use hazers indoors. A hazer works by
making very tiny particles of medical grade oil. These are small enough to be
flushed out of your lungs by normal breathing and if properly set up, are
odorless and OSHA approved. Fog machines for the most part are crackers, they
work by incomplete combustion of glycols (aquafog) or burning of oil in air.
Hazers fragment the oil in CO2 and thus are almost odorless. Plans for a
homemade hazer of sorts that uses air are at
LaserFX on the
"Backstage" pages. It has a slight odor but is not that bad to be around, and
mind you I have asthma! I have done indoor shows for 1,200 people using 60 mW
and a cracker. I have also done shows indoors for 100 people with a 5 mW hene,
it depends on ambient lighting and air circulation/humidity.

It is a minimum of about 5 watts of argon light for a decent outdoor smokeless
beam show, with 20 watts being more typical.

(From: Steve Quest (Squest@cris.com).)

Visible wavelength lasers are more visible in 'plain air' if the angle of
incidence is low (you're close to the same angle of the beam) and if the power
is greater than about 5 watts. I perform an outdoor laser show using a 30 to
57 (max) watt YAG (frequency doubled to 532 nm) which is plainly visible in
mostly clear air (no need to smoke, or fog the air). When I want to do beam
effects with a 5 watt argon/krypton white-light laser, I have to fog the air
up.

Plain outdoor air has enough particulate matter to scatter a laser beam so
long as it is above 25 or so watts, thus making the beam visible. Of course,
the more power, the brighter the beam looks, but CDRH has limits, and that
limit is .9725 mw/cm2 at 750 feet, so the days of power beam shows
going all the way to outer space and beyond is over :-(.

I use a Laserscope laser, which is FDA (Food and Drug Administration)
approved, and am following CDRH (Center for Devices and Radiological Health)
guidelines, receive FAA (Federal Aviation Administration) approval and air
clearance before every show, and make sure that NOTAM (NOtice To AirMen) are
issued to pilots flying in the area of my shows, giving exact details as to
what is going on. Pilots love the shows, and air traffic routes planes WAY
out of their flightpaths to fly near the beam shows to get the best seats in
the house. :) However, I have to beam-off when they get too close, then they
return to their flightpath, and I can resume the show.

I used to be able to sparkle off the new moon with my YAG at full power and
full convergence. It takes some doing but you can see the sparkle from the
Sea of Tranquillity with the naked eye off the corner cube reflector, aka:
retroreflector left there in 1969 by the astronauts.

(From: Sam.)

WARNING: Shooting a laser into the sky is irresponsible and highly illegal
without prior approval from the proper agencies. Airline pilots do not
appreciate being blinded!

Here are some additional comments on the effects of viewing direction on
apparent brightness:

(From: Johannes Swartling (Johannes.Swartling@fysik.lth.se).)

What you see is light that has been scattered by the small particles in
the fog or smoke. This kind of scattering is called Mie scattering, and
occurs when the size of the particles is comparable to or a little
smaller than the wavelength of the light. In Mie theory, there is
something called a scattering profile - i.e., the probability that the
light will scatter in a certain direction.

Now, in the case of very small particles, such as molecules, this
scattering profile is isotropic. That means that the light will scatter
in all directions with equal probability. This special case is called
Rayleigh scattering, and can be seen from pure air if you have a strong
enough laser, such as an Ar-ion laser. When the particles get larger,
however, the light will tend to scatter more and more in the forward
direction. That is what you see from the smoke. When you look along the
beam in the direction where it comes from, you see a lot of light that
has been scattered just a little bit off the direction of the beam. When
you look along the beam away from the laser, there's a lot less light
that has been scattered backwards.

(From: Pissavin (pissavin@aol.com).)

One interesting phenomenon; Depending on whether dust or smoke is used, there
is an asymmetry: With smoke, if you put your head near the laser and look
down the beam, you see almost nothing. Now, look toward the laser (BUT NOT
DIRECTLY INTO THE BEAM!) and you see a clear beam. Then replace the smoke
with dust and the effect will be reversed.

(From: NeoLASE (neolase@lasers.org).)

Large particles like dust have more back scattering centers while small
particles like smoke and haze have more forward scattering centers.
Mie scattering effects, and all that stuff, I've heard/read of but I
haven't studied in detail. Used a lot in laser particle size analysis.

For the most power available, usually a krypton ion laser running red only and
an argon ion laer for the blue and green is combined. The krypton red
wavelength (647.1 nm) is not the best for color combination for true RGB
mixing but it is about all that is available with adequate power. Remember,
even if the argon were to produce 20 watts evenly split between green and blue,
and 10 watts of red from the krypton, a total wattage of only 30 watts is
available for the entire picture area. This really isn't that much for a
large scale presentation and is why Vegas uses RGB light bulb boards as well
and stadiums use Jumbotrons or Diamond visions, not lasers. The total light
available is 1,000's of times brighter, and even with coarse resolution, the
distance from the screen blends the image. Raster scanning with a laser is
very inefficient, but with vector scanning and raster some unique effects can
be created. Better yet use 10,000 watt lamps, one for each color via the
proper filtering and use light valves to control the each device for each
color. Like a projection TV except on a huge scale. And cost is always a
factor.

How well this works depends on the pulse rate and pulse width of your laser
and how fast you are scanning, and how much you like dots and dashes
in your image. It also depends on how you are shaping your image - i.e ,,
some non-galvo imaging systems use pulsed YAGs for projection video.

However if you are talking about an AO Q-switched YAG at a high rep rate, you
can do, say, 10 to 12K galvo graphics. It just shimmers a lot and has faint
spots that wander through the image. The real killer is that the divergence of
pulsed YAG lasers of any significant power is extremely high and when the
divergence magnitude starts to catch up with the resolution of the points in
the image, you get a blob. When it catches up with the scan angle, you get a
bigger blob. This happens at say a couple of hundred feet from the laser.

I have witnessed this as a member of the crew on a show using a Q-switched
YAG for beam effects. The company owner wanted to try scanning images on a
building some distance away to see how his collimator worked. Up close it
wasn't bad. But, more then a hundred feet or so from the laser, it was "The
Green Blob".

Being able to project a 3-D image hundreds of feet into open space is pure
science fiction - there is no current technology and even basic theory that
would make this possible without some medium to act as a screen. However,
some pretty vivid illusions that may give the impression of such a display
do exist and you may experience one at your next large scale laser show:

(From: L. Michael Roberts (NewsMail@laserfx.com).)

The most common way of creating this illusion is to use a scrim or a water
screen. The scrim is a thin fabric screen, like mosquito netting, that is
often dyed black or dark grey. It is rolled/lowered/flown into place while
the audience is looking at something else, then used for laser graphics
projections. Using typical modern 30K PCAOM projectors, flicker free
images can be projected onto the scrim. While most of the laser beam goes
through the scrim, enough of the laser is intercepted and reflected by the
threads in the scrim to form an image.

The water screen is a similar concept except that it uses a thin film of
water droplets sprayed into the air as a projection surface. Both there
techniques allow one to create the look of an image suspended in mid-air -
especially if the audience is fixed in relation to the projection surface.

There is a beam interference technique in the early stages of development but
it isn't likely to ever result in a large scale display out in open air. It
was pioneered by Dr. Elizabeth Downing. The image is generated inside a
specially doped glass cube using scanned IR lasers. At present. the display
s barely 2" on a side. For details see
3D Laser Based Volumetric Display.

I am fairly new to lasers (been studying and researching on internet for
about 1.5 years now, especially Sam's Laser FAQ) and decided a few months
ago to do my own laser show for our New Millennium eve party. We had about
30 or 35 people in attendance, and a musical show that lasted about 40
minutes. The equipment consisted of a home built Lissajous pattern
generator (not the spinning motor kind) with laser modulation driving a
GAL-2, a 1 watt stereo audio amp with raw audio from the show music
driving a GAL-2, and 2 lumia wheels with 3 lasers shining through them.

All this was projected on a silver screen (plastic tarp) suspended about
15 feet above the audience (no audience scanning done of course) . The
lasers were all laser pointer types with the batteries removed and wires
attached, and all connected to a home built laser power control station
which controlled power to each individual laser. Fog beam effects were
accomplished by spraying 'fog-in-a-can' at the beams. It turned out
great, with all who attended enjoying it very much (granted, most of
them had never seen a 'real' commercial laser show).

It was a really fun project and will be done again at years end this
year! I would encourage anyone who might be thinking of doing this to
go for it! It was not really expensive, and was worth every penny for
the all around experience. I also included my son (who was way better
than me at operating the pattern generator) in the show, so he got a
real kick out of it too. Highly recommended!

(From: John Craker (watts@dccnet.com).)

I built a basic laser show from a dead (semi dead?) LaserDisc player. When
hooked up to my home stereo, it displays lovely (and useless) Lissajous
patterns on my ceiling.

I basically robbed a section of the chassis that housed the HeNe laser and
another section that had two deflection mirrors. Pointed the output of the
laser into the mirrors. I hooked up the coil of each mirror to each channel
of my stereo. With the difference in the stereo signal, you have each mirror
oscillating at a slightly different rate, and since one mirror deflects in
the 'Y' axis, and the other in the 'X', you get this great ever changing
display. Size is pretty much adjusted via the volume. :)

(From: Sam.)

Based on a photo that John sent me as well as the sample in
Optical Pickup from HeNe Laser-Based LaserDisc
Player, the deflector from this LaserDisc player
would appear to be virtually identical to what Meredith Instruments
used to sell as GAL-2 (I don't think they have them anymore).
I wonder if that's where they got them. In the LD player, the galvos
were used for fine tracking and tangential (timebase) correction. I also
have seen similar deflectors in other somewhat newer Laserdisc player optical
pickups. However, if an IR rather than a HeNe laser was used (as would be
the case with anything after about 1983), the
mirrors will likely not be highly reflecting at visible wavelengths
(though very thin mirrors could perhaps be glued on top of the
IR mirrors, with a slight sacrifice in performance). Along
with a HeNe laser or laser
pointer, and low power audio amp, you're in the instant light show
business. Well, at least for those boring Lissajous patterns! :) The
GAL-2 is sensitive enough to be driven by a personal stereo but the 4
ohm input impedance may overload and kill its output amp if it is
designed for 32 ohm headphones.

Note that while the GAL-2 (or LaserDisc deflector) appear
superficially similar to a pair of loudspeaker voice coil/magnet assemblies,
the pole pieces of their magnets are on either side of each coil rather than
within and surrounding them as in a true loudspeaker. Thus, the coil, and
thus mirror, pivots from side-to-side as expected and desired rather than
moving in and out.

If you are conducting high-precision scientific experiments, or doing
holography, you will need one of the BIG (4 x 8 foot (1.2 x 2.4 m
approximately), vibration isolated optical tables like the ones available from
Melles Griot, Newport, and others. You will also need a large wallet, not
to mention a solid foundation and space to locate it!

If you are a starving laserist who wants to make something for mounting
lasers, a few optical components and scanners, you can semi-DIY for a
reasonable cost.

Go to your local machine ship and have them order a sheet of 3/8" (9.5 mm)
T6016 aluminum large enough to mount your laser(s) with space left
over. I would suggest 5' (1.52 m) by 18" (0.46 m) for medium frame
lasers, longer if you want to build a beam table. Now have the
machine shop drill and tap 1/4-20 holes on a 1" grid (M6-1.0 holes on 25
mm grid for metric).

To save money, do not have the entire plate drilled and tapped. Leave
the area where you intend to mount the laser(s) blank - you could have
the shop put in the mounting holes for the laser(s) or you could do it
yourself. The area where you intend to mount the electronics can be
drilled and tapped on a 2" (50 mm) grid. You will need some mounting
holes in that area, but unusable holes under transformers and PSUs just
cost money. The area at the output of the laser(s) should be drilled
and tapped with the full 1" (25 mm) grid as this is where you need the
most flexibility for mounting optics.

When the plate is done, have them chamfer the edges and send it out for
black matt anodizing. 4 or 5 years ago, I could have one of these made
up for around $250 CAD - YMMV

One last tip: When choosing the mounting position of the laser, make
sure the output beam will fall between two lines of holes, and parallel
to the holes, in the grid to allow the most flexibility on mounting
items on either side of the beam.

If you just want to get your feet wet in laser shows, you can mount small
first surface mirrors on a pair of loudspeakers. But to get the full effect,
they have to be modified to alter the angle of the mirror, not push it back
and forth. An alternative is to mount mirrors on small hobby motors at a
slight angle. Up to 3 motors can be used with the beam bouncing off the
mirrors in succession. Varying the speed of the motors individually will
then produce a large variety of Lissajous and/or spirograph type displays.

One step of from these bare bones approaches is to obtain a set of inexpensive
galvos. The most common source for these are from ancient HeNe laser-based
LaserDisc players. See the section:

Laser
Show on a Shoe String. However, if you want to play with the "big guys",
then what's needed is a pair of high speed galvos and the driving electronics.
These are probably even more important than the laser(s) in determining the
ultimate performance of any laser show:

(From: Steve Roberts.)

May I suggest what I suggest to all beginners in Laser Shows?

Buy a copy of L. Michael Roberts' (no relation) book on Laser F/X, its
well worth the money and will save you much reinventing of the wheel.

Save up and buy decent scanners, much fun can be had with the slower
stuff, such as G330s, but in the long run you will find yourself needing to
acquire faster scanners anyway and will be setting yourself back financially
and time-wise with the slower units.

Stay away from General Scanning G120s unless you get a complete system
tuned by a professional. There are many abused and rebuilt G120s out there,
a source of heartbreak when they fail easily. The scanner amp tuning on them
is very critical, else they can oscillate and break.

Medialas Catweazles - Very hard to blow up yet nearly full professional
performance.

Cambridge 6800 or 6210 - These are the holy grail but they are very
expensive

Don't bother with galvos like CECs - they are designed for exposing beams in
small chart recorders using a ultraviolet arc source, they are referred to as
"pen" galvos, and thats what they are, about the size and shape of a ink pin,
with a small mirror about .5 mm across. They are thus too small to make a XY
mirror pair, especially since the external magnet needed is huge.

I've been looking into installing a lens in my projector to increase
the divergence enough to make audience scanning safe. I've seen
recommendations to use -3 diopter lenses in small rooms, but I wanted
to be able to calculate appropriate strengths for different
conditions. So here goes:

Lens strength is often specified in diopters, which is simply the inverse
of the focal length, that is:

D = 1/f m-1

Where D is the strength in diopters and f is the focal length in
meters. (A nice property of diopters is that lens strength is
additive - the result of using lenses D1, D2,...,Dn is simply their
sum D1+D2+,...,+Dn.)

A collimated beam that passes through a lens will get a half-angle
divergence of:

θ = r0/|f| = r0*|D|

Where r0 is the radius of the collimated beam. Note that it's the
absolute value of the strength that is used. A concave lens will give
the same divergence as a convex of the same power. However, the convex lens
will have its focal point in front of the lens, which is not
desirable. It is also interesting to note that the divergence is
directly proportional to the beam diameter, which means that the
initial size of the beams is very important and that they must be
matched.

The size (radius) of the beam at a distance z is given by:

r = r0*z/|f| + r0 = r0*z*|D| + r0

For example, using a -3 diopter lens to diverge a beam with a 3 mm
diameter and measuring at 4 meters:

r = 1.5 mm * 4 m * |-3| m-1 + 1.5 mm = 19.5 mm

Which means that the beam will be 39 mm wide at 4 m. Assuming a
circular beam with a flat profile (not very realistic though) the
following equation can be used to calculate the irradiance, which is
really what matters from a safety perspective:

E = φ/A = φ/(2*π*r2) = φ/(2*π*r02*z2*D2)

Where E is the irradiance (W/mm2) and φ is the radiant flux (W).
To convert to W/mm2, multiply by 100. While the calculated
values here should be taken with a grain of salt because beam profiles
are never flat, it is useful to see that doubling the strength of the lens (or
distance) gives 4 times lower irradiance. The actual irradiance must
be measured to assure safety.

Green and Blue are generally produced by either a multiline argon ion
laser (though a DPSS laser is often replacing the power hungry ion
laser for green at least). However, getting high power red requires
either a krypton ion (or mixed gas) laser or very expensive DPSS laser.
Even the largest HeNe laser (SP-125, multimode if one exists) won't
break the 200 mW barrier and it's very difficult and costly to get
decent beam quality from a red diode laser. Orange and yellow are
at least as much of a problem. So, what about pumping a dye laser
with an argon ion laser?:

(From: Steve Roberts.)

As an example, a Coherent 930 medical system uses a modified
I90 tube with a CR599 three mirror dye head. Threshold for
the dye from the factory docs with fresh R6G, fresh optics, and a good
tweak, is 1.5 watts all lines from the argon, lasing at a few
milliwatts tuned at 640.2 nm. Note that the power is only about 40 mW
at 2.5 watts pump, reaching a max of 3.2 watts with 9 watts pump. The
specially selected MRA tube with extreme multimode optics reached 12
watts when new at 40 amps, but was designed to only sustain these
powers for 30 seconds or so at a time. The opthalmologist or dermatologist
who would use one of these needed about 0.7 watts of treatment power.

2.5 watts pump is about 24 amps down the tube.

If you moved the unit around without draining, the dye reservoir vents
are set up in such a way that you would leak dye solution into the
PSU. There is no drain on the unit, you'd have to suck it out. The
dye solution is not just methanol, it has some nasty additives to
quench triplet states that prevent the dye from lasing, the dye
pressure is about 40 to 150 psi adjustable and it squirts across a
air gap.

I still have the R6G stains on the garage floor from scrapping a
aurora dye a few years ago.

Now if you have room for a second three-phase laser (in addition to
your green/blue argon ion) laser at your rave and a large box truck
with lift-gate, don't mind a 400 pound 6 foot long 18" wide console
on wheels (build the beam table on top of it!) and like cleaning
liquid cancer off your optics while ruining a change of
clothes every time you open it up, then this is the laser for you.
Splitting it into boxes would cost a lot as the linear PSU is
spread out all over the thing. If you run it at 2 watts of tunable
red through yellow it would be a hell of a show, especially if the
stepper controller on the tuner was rewired. If the tuner is
removed, it would lase broad-band by a few nanometers at the
peak of the dye.

By the way, the blue pump beam is nearly totally adsorbed if the thing is
tuned to rock, and fluctuates and sputters like a lumia on the wall of
the dye head.

Laser Based Systems for 2-D and 3-D Display

I am sure everyone has heard of the predictions that there would be mural (or
stadium) sized TV screens using lasers instead of the other silly technologies
like LCDs and light valves. This was 10, 20 years ago. Where are they? The
idea is simple: Replace the three electron guns in the color CRT with red,
green, and blue lasers and raster scan a TV picture onto your favorite screen,
barn, or mountain-side. :-)

There are now companies marketing (or at least seriously demonstrating) laser
based TV displays. The most recent versions use a single multi-color diode
pumped solid state laser. One such unit has an optical output of about 13 W.
To put this in perspective: The visible output of a 250 W incandescent bulb
is about 13 W. So, that's a lot of light for a small screen but isn't going to
compete in a theater setting. And, you don't want to ask about the cost! :)
But, see the section: About the Schneider High
Power DPSS RGB Laser/Projector for info on one such unit.

Using laser diodes directly rather than solid state lasers has some fundamental
problems. The first has to do with color. Untill recently, you could have
any color of laser diode you want as long as it is red. :) While moderate
power (perhaps up to 500 mW or 1 W) red laser diodes have been around for
awhile, laser diodes with an actual blue wavelength (430 to 445 nm as opposed
to deep violet - around 400 nm) are just becoming available as costly
engineering samples with all sorts of strings attached and they have
power outputs of only a few 10s of mW at most (see:
Availability of Green, Blue, and Violet Laser
Diodes). Of course, even 445 nm is more violet than blue, 460 would be
better, but it's a start. Green laser diodes aren't even on the horizon
in an commercial form and those tested in the lab have had very limited
life and may have operated only at cryogenic temperatures.
Unfortunately, even if high power RGB laser diodes could be purchased for $10,
due to the fact that they would operate with multiple spatial modes along one
axis, generating a tightly collimated beam suitable for direct scanning would
be very complex and expensive, if not outright impossible. Better go to
plan B. :)

However, there is what might be described as a hybrid technology that
still use lasers for the light sources but with a MEMS (Micro ElectroMechanical
System) for the modulation. The Grating Light Valve (GLV) is a 1 dimensional
array of MEMS-controlled diffraction gratings. See
Silicon Light Machines Products and Technology. A typical
system for TV or computer display would utilize 3 GLVs (one for each primary
color). Each GLV would have enough channels for a vertical or horizontal
line of the display and conventional (low speed) mechanical deflection such
as a galvo would be used for the other axis. Such systems have been
demonstrated and the GLV already has a track record in the printing
industry where it is used to expose master printing plates a swath at
a time using a high power IR laser diode line source. While there are
no fundamental technical problems with this approach and it is certainly
much simpler in some ways than direct scanned laser TV, there is still the
not so minor issue of low cost high power lasers. But at least, multimode
diodes can be used so when high power blue and green laser diodes are
available, we'll be all set. :)

(From: James A. Carter III (jacarter3@earthlink.net).)

Just to let folks know where this Laser TV thing has been.

In the 1920's, a company in England, Scophony Labs (I think that's right)
patented a method for using Bragg diffraction on tanks of water (that's right
H20) to display TV signals using white light (thermal) sources. They had to
use BIG beams because they didn't have lasers. BIG beams mean low modulation
rates due to acoustic transit time. Their idea was to scan the spot so that
the acoustic pulse was stationary on the screen. I believe that they didn't
use galvonometric scanners for the horizontal scan, instead they put mirrors
on motor shafts (similar to what some cinemagraphic projectors used at the
time). The scan rate and magnification were selected so that the scan velocity
vector was equal and opposite to the image of the acoustic velocity
vector. This may have been an idea way ahead of its time.

Just ten years ago, I helped design the optics of a system that does display
not only NTSC images but scan to HDTV as well. This is not a cheap system and
is certainly is not suitable for avionics; although the Air Force (through
TRW) did buy many systems. It used an air bearing motor to drive a many
faceted polygonal mirror scanner for the horizontal scan and used a "galvo"
scanner for the vertical. The AO modulators had enough band-width (at least
500 times what you get from PCAOMs) to project NTSC images in a flying spot
mode. That is the scanner was going much to slow to give the Scophony
condition. When we ramped the system (it was a closed loop continuous
multiscan projector) to 1280 by 1024 sources, the scan was fast enough that we
achieved the Scophony condition and realized over 35 MHz of video bandwidth
per channel. This is somewhat inadequate for computer CAD graphics but was
quite acceptable at the time. The display was dazzling, to say the least. Per
laser color for each red, green and blue channel with red at a deep and rich
635 nm (dye laser pumped by the otherwise useless cyan lines), and the argon
lines for green and blue. We used a 10 watt argon from Spectra-Physics to be
the photon engine (SP was an investor here). One of these went to the NAB show
and displayed our beloved President Ron.

Unfortunately, the lasers were not reliable enough, to expensive to repair and
replace, and more light is always better. Further, the big guys (TRW and SP)
started to bicker and the company went under. The last time I saw one of
these systems was at SP Corporate in San Jose. I was there to install a 25
watt laser, but that's another story.

Current commercial work centers on dumping the high speed scanner and using an
AO cell to modulate the whole line at one time. Bragg cell technology can give
the Time-Bandwidth Product (TBP) required which is certainly over 1000 and
closer to 2000. Unfortunately, acoustic attenuation (Beer's law in time and
space) and the non-uniformity of the laser source (typically Gaussian) require
losses to make a nice uniform display. Even with HIGH power pulsed lasers
(repping at the horizontal line rate or at a multiple), the display can lack
luster.

As always, more photons... more photons...

(From: Tony Clynick (tony.clynick@btinternet.com).)

I am pleased to tell you that laser video projection is still very active
in the UK. Based on the original laser video projector (LVP) made by
Dwight-Cavendish in the early 1980's, the projector now made by the team at
LCI (Laser Creations International in London) has been installed at several
permanent sites in theme parks since 1994, mostly in East Asia, and has been
used for dozens of temporary shows world-wide since 1987. Most applications
are in exhibitions, outdoor shows and theme parks.

The LCI-LVP uses SP white-light lasers with special optics to provide good
flesh-tones so the need for dye lasers is eliminated. A polygon scanner (GEC
Marconi - thanks Alan) provides the line scanning, at rates of up to 36kHz.
AO modulation and Scophony balance provides video bandwidth up to 30MHz, so
HDTV (1250/50 and 1125/60), as well as PAL/NTSC/SECAM are available in the
LCI-LVP. Output on screen of a peak-white modulated raster of over 15 watts
has been achieved. The largest image projected so far was 50 metres wide.
The collimated scanned beam provides an infinite depth-of-field, which was put
to good use last year at the Singapore National Day on a giant 35m x 28m
high-gain screen laid over the slanted stadium seating. The difference in
projection distance between the top and bottom of the screen was nearly 100
metres, so the LVP was the only machine capable of a focussed image over the
whole screen. All LVP's supplied so far by LCI are also capable of vector
scanning using the waste AO beam.

(From: Chris Cebelenski).

I know of one experimental project that uses an array of galvo's to project a
raster image at 1/2 normal NTSC refresh rate (15 fps). The cost of this
endevour so far has been, well, let's just say it's been expensive. :-)

Each quadrant is then split into separate red, green, and blue channels.

Each channel goes to a separate GS 6800 scanning head and PCAOM to form
addressable RGB "pixels". It was done this way before 6800 scanners were
available and because the decoding circuits were easier to design (at the
time). I would probably do it differently todays.

There are several problems with this:

Size. Stadium sized projections are fine, but it doesn't work too well in
a dome. U2 would love it. (Search for: "Popmart tour" on altavista).

Cost. Enough said!

Power loss. Even the 5 W laser can get dim. With some mods it could work
with multiple large-frame lasers, but then there's #2 again.

Unlike most laser systems, it works best when projected against a BLACK
background. White backgrounds have much better reflectivity, but the image
really doesn't look right due to bleed and scatter.

Max and min sizes - make it too large and it breaks up and the scanners
can't keep up. Too small and the resolution of the scanners isn't good
enough to provide a clear image and cross-talk is rampant.

(From: Steve Roberts (osteven@en.com).)

Two years ago I was at a Laser-FX conference in Canada, we had the chance to
watch (I have it on tape) a Russian made scan system with no moving parts, all
acousto-optic and almost totally analog driven, that produced sharp clean
monochrome images without flicker the size of a billboard using a 6 watt 532
nm YAG . The marketing person explained that RGB existed in the lab and was
not far away. I believe the company name was Lasys Technologies. Scan head and
laser was about the size of a PC/AT case and sat on a tripod, and was easily
handled with low weight. Ran off 220 VAC three-phase, but I was told 220
single-phase would not be a problem. Further details can be obtained from:
L. Michael Roberts (lmichael@laser-fx.com) who was the organizer of the
conference.

(From: L. Michael Roberts" (NewsMail@laserfx.com).)

Some of the newer laser based video projectors (e.g., the Samsung unit) use
a white light laser (Ar/Kr) as the source - 3.5 to 10 watts depending on
the image size and brightness desired. The beam is split into it's prime
component colours, modulated, recombined and then scanned.

Many of the older units used a tandem laser pair - an Argon and a red-only
krypton. Some units even use three lasers - an argon with blue optics,
and argon with green optics and a red-only krypton. This takes a LOT of
water and power to operate.

There is presently a lot of work being done on producing compact diode
pumped YAG based red and blue lasers. Laser Power showed prototypes of
these lasers at the ILDA meeting in Amsterdam last November. This would
allow people to build a fairly powerful (2 watts input approximately)
laser based video projector that is air-cooled and can run on 115 VAC.

(From: Sam.)

Here is a link to an article about a system that may be commercially viable
in the near future. It uses second and third harmonic generation to produce
green (532 nm, 13 W) and blue (447 nm, 7 W) output, respectively, from a pair
of Nd:YVO4 diode pumped solid state lasers along with a diode
pumped optical parametric oscillator to generate the red (628 nm, 10 W) beam.

The company claims their market advantage to include higher resolution (1,600
x 1,200) and better contrast ratio (1,700:1) than competing non-laser based
technologies. They also cite lower maintenance than arc lamp based systems.
However, the cost is also much higher at present and I question the brightness
of 3,000 lumens at the screen (this is about equivalent to the total light
output of a pair of 100 W incandescent bulbs) so it may still be inadequate
for theater-size applications.

And, here's a description with photos of a laser TV system built back in 1985
(along with some other related laser display gadgetry):

One thing that is nice about TV using lasers is the use of a true red
"gun". I've built 3 or 4 different versions of laser video projectors using
argon and krypton lasers and the first thing you notice when you put a standard
color bar signal up is that it looks "different". The reason is that in normal
television there really is no such thing as a red phosphor. They are actually
closer to orange than red, but by color mixing and a little fooling of the
brain, you see red from the orange phosphor. So when you finally do see
a video display that comes from a fairly dark red line (like the 650 of the
krypton), things that normally look really bland like browns, violets, and
other colors that depend on red, look stunning. It makes normal television
look much more like film that video. Oddly enough, a couple of commercial
laser video companies went to great lengths to produce the orange line instead
of the red from a krypton by using argon pumped dye lasers to produce the
orange. I could never, ever figure out why go to such trouble except that they
were so anal about trying to follow NTSC standards for color that they
ignored the benefit of having a true red. I had a little secret method for
curing those situations where the client would complain about the color and I
could give them orange back without the use of the dye laser, but normally
once they saw real red, they wouldn't let you touch it. It makes sense, most
color CCD camera (at least with three CCD's) use color dividing prisms that
cutoff into the red more than orange.

Displays capable of providing information about the three-dimensional aspects
of a scene can be divided into two classes:

2-1/2D displays where the 3rd dimension is supplied by a variety of cues
that allow us to perceive depth on a conventional 2D CRT of other similar
device. Such cues include shading, motion parallax, multiplexed images (where
a mask permits the viewing position to determine which of several images is
visible, and the use of stereo (two views, one for each eye). However, none
of these produce a true 3-D view which it is possible to change your viewpoint
or that of a camera's lens left, right, up, down, forward, back, and perceive
or record the geometrically correct image. In other words, while these 2-1/2D
techniques do work - and quite well and adequately for many applications, they
are not true 3-D.

The advantages of these approaches are that they are well within the
capabilities of modern digital processors and display devices.

Volumetric displays actually attempt to physically recreate the 3-D scene
using some sort of 3-D scanning or vector drawing technique to enable each
voxel (the 3-D analogy to the pixel) to be selectively activated.

There are various technological hurdles to be overcome to make this sort of
display practical since with a 3-D volume, much more data needs to be
rendered and transferred to the actual display hardware. There are also
fundamental problems with implementing hidden surface removal.

Holographic displays are for the most part still in the future (though
there may be exceptions as noted below). These capture the entire 3-D
information of a scene using holographic techniques and render the resulting
interference patterns on an ultra-high resolution display device like an
LCD panel with a 100,000 x 100,000 pixels and 100 GB/second processing and
loading speed!

Needless to say, just a bit of work needs to be done before one of these will
be as inexpensive as any TV set. However, see the section:
Holographic Video Displays.

There have been a number of volumetric (not true holographic) displays
developed over the years using rotating mirrors, disks, LED arrays, disks
inside cathode ray tubes, etc. These are all scanned in such a way as to
cover a true volume of space at a rapid enough rate (at least that is the
objective) to produce the illusion of a solid 3-D volume floating in space.
The scanning source can be a laser, electron beam, or the projected output
of another 2-D display like a CRT or LCD panel.

Currently, there are technical issues to be resolved with respect to the
bandwidth of the channel to get the information into the display
(Gigabytes/second are required for adequate refresh rates). But more
fundamentally, these techniques are incapable by their design of rendering
solid shaded surface views. The volumetric display is one of 'look through'
or 'structured fog'. However, such a technique in a practical application
could be extremely useful.

With technologies as yet unavailable, one could conceive of a 'selective
activation' display where points in 3-space are rendered opaque or emissive
by intersecting Laser beams or something like that. There has been progress
in this area with emissive displays - intersecting laser beams resulting in
the production of colored points of light. However, all these technologies
suffer at present from serious resolution and bandwidth limitations - not
likely to be solved for decades at least. (See below.)

A true holographic display would be capable of an ***arbitrary*** viewing
mode including the display of solid surfaces with shading which would
be viewable with correct perspective and shading from a range of angles.
I do not know of any actual examples of such technology at present. An
emissive volumetric display like the one described below cannot implement
hidden surface removal - essential for life-like rendition. While wire-frames
and look-through displays have many uses, they aren't likely to be of much
value for a boob-tube replacement! :)

Already in the works! A "Three-Colour, Solid-State, Three-Dimensional Display
based on two-step, two-frequency upconversion in rare earth doped heavy metal
fluoride glass is described. The device employs infrared laser beams that
intersect inside a transparent volume of active optical material to address
red, green, and blue voxels via sequential two-step resonant absorption.
Three-dimensional wire-frame images, surface areas, and solids are drawn by
scanning the point of intersection of the lasers around inside the material.
The prototype device is driven with laser diodes, uses conventional focusing
optics and mechanical scanners, and is bright enough to be seen in ambient
room lighting conditions.

That's a block of (expensive) glass with some lights in it? Last thing I
heard, they'd only got a low resolution. But a couple of years ago I was
at a Philips trade show. There was a true (?!?) 3D laserTV system. In a
room, a music video was shown. There were a number of layers displayed
in air (fog?) so you'd get a 3D view. Nice thing was that you could walk
right through the image and still see it. But I've never heard of it
again. Anybody knows if they're working on this now?

Examples of art pieces made under computer control of a pulsed laser focused
inside a glass block can be found at
3D Laser Art Co.. They
have a basic explanation of the process but no specifics and no mention of
the type of laser that is used.

(From: Steve Roberts.)

Engraving inside a block of glass is a pretty easy thing to do if you
have a high power pulsed YAG laser. I've seen problems in labs with cheap
glass lenses developing spectacular defects in the middle of the glass, so a
variable focus lens, some galvanometer scanners for positioning, and a monster
pulsed YAG - plus some decent software and you should be able to carve in
flint or lead glass.

It's all too easy to create microcracks on the insides of the cheap lenses.

(From: David Toebaert (olx08152@online.be).)

The December 1999 issue of 'Laser und Optoelektronik' has a beautiful picture
on the cover of a piece of lead crystal with the Dresdner Frauenkirche inside,
3-D engraved using Nd:YLF (Q-switched AND mode locked) lasers. It was
developed by the Fraunhofer Institut fur
Werkstoff- und Strahltechnik.

(From: A. E. Siegman (siegman@stanford.edu).)

The basic process is a.k.a. "bulk (or internal) optical damage" produced by a
focused laser beam. The basic effects were observed with the very earliest ruby
and other pulsed lasers in the early 1960s, very often unintentionally and to
the detriment of expensive optical components including sometimes the laser rods
themselves. This led to a whole field of "laser damage" studies, including a
series of NIST-sponsored symposia and other publications over the next several
decades, and quite a lot of early work in the Soviet Union also..

The physical process involves a complex mixture of photoionization, multiphoton
ionization, melting, vaporization, and various stimulated scattering processes,
leading to bubble formation, track formation, and "micro-explosions" occurring at
either the focal spot or at various intrinsic defects inside the material. The
exact details of what happens depend on the wavelength, intensity, and pulse
duration of the laser pulse and the physical characteristics of the material.

There are a number of small firms in the U.S. and elsewhere who will write the
kind of decorative cubes you saw in the gift show, in glass or plastic cubes,
using computer-controlled pulsed YAG or other lasers. They will also fabricate
inexpensive customized versions as mementos for going-away parties, bowling
trophies, and so forth.

There is also a recent (late 1990s) patent by a British guy on a subsurface
marking apparatus of this sort which has been used by a major distillery for
writing subsurface serial numbers into the bottoms of zillions of Scotch whiskey
bottles. I'll not provide a citation because IMHO given the prior art and state
of knowledge of these effects the patent should never have been issued.

(From: "Beric" (beric@ntlworld.com).)

Its actually British Technology, but as usual developed overseas. The patent
is owned by United Distillers. They are micro cracks, that are laser written
into the glass.

Introduction to Holography

Holography represents a class of techniques which capture 3-D information about
a scene as an interference pattern on or in an extremely high
resolution 2-D film. When the film is developed and viewed under the right
conditions (some require a laser for viewing while others can use a suitable
white light source), the result is a recreation in every detail of the original
including the ability to move your viewpoint and look around objects, proper
hidden surface removal (solid objects appear solid), shadows and highlights,
and so forth. In principle, the hologram is optically indistinguishable from
the original. A normal photo of a hologram would look the same as a photo of
the scene itself.

However, in so far as the technology exists today, holography is NOT what is
often depicted in Sci-Fi and other movies and TV shows. Some of this
deficiency is due to fundamental principles of what holography is and how it
works while much of it is due to the inadequacy of present technology:

The hologram itself (film or plate) must be in the view. In other words,
it isn't possible to project a hologram into empty space and view it from the
side. With a suitable setup when the hologram is created, the perspective
can be arranged such that the scene *appears* in front of the film but it must
be bounded by it. The effect in this case is that of looking into a window
but with the scene appearing in front of the window based on depth cues.

The quality of holographic reproduction is currently far far below that of
conventional photographic techniques simply because so much information must
be captured on film with limited resolution and almost everything else affects
the final outcome including the slightest vibration (on the order of a
wavelength of the light used) and even air currents in the studio.

Although progress is being made constantly, there are still much that needs
to be done to perfect the quality and resolution when viewing holograms
without using lasers and in producing true full color holograms (not the
'rainbow' holograms seen on credit cards and logos).

(Portions from: Rick Poulin (rpoulin@rohcg.on.ca).)

While holography is really still in it's infancy it already has many other
fascinating applications. Just a few of these include:

3-D representations of rare and delicate artifacts, carvings, and
paintings. By capturing every nuance of the original, the actual object can
be put away for safe keeping under ideal conditions of temperature, humidity,
darkness, etc.

Analysis of vibrational modes or deformation due to stress on a
microscopic scale. A hologram of the object being tested is made in the
usual way. Then, without moving the object, the developed hologram is
replaced in exactly the same position as it had originally and the laser is
turned back on. Any displacement of the object's surface - even on the scale
of a fraction of a wavelength of the laser light being used - will show up as
a visible interference pattern. This is being used to evaluate everything
from priceless 300 year old violins and jet engines to scale models of
proposed buildings, bridges, and other structures subject to vibration or
stress.

Multiple 2-D images from a medical CT or MRI scanner may be stacked by
recording them successively onto a single hologram. This is done by
displaying the orignal slice data using a 2-D Spatial Light Modulator (SLM)
like an LCD panel which becomes the 'object' for a normal hologram. After
each exposure, the SLM is moved a small distance relative to the holographic
film and the next slice in the sequence (separated by the slice thickness) is
displayed and recorded. When the film is developed, the result is a 3-D
'reprojected' or 'look-through' image with anatomy appearing semi-transparent
that is more meaningful to someone not trained to read 2-D scans or normal
X-rays 'plane' films. By preprocessing the images on a conventional medical
workstation, structures of interest can be enhanced and others can be removed.
These "Voxgrams" (from voxel - the 3-D analogy to pixel) can be used for
surgery planning or to explain a problem to the patient! (Note, however,
that this is not the true 3-D possible using holography directly.) Using
advanced techniques, the hologram may be viewed with a simple low cost white
light display box without requiring a laser. See:
Holorad: Holograms from CT and MRI Data.
(Unfortunately, while the technology works and is impressive, VOXEL, the
original company, was just a bit overly optimistic about their ability
to develop and market a holographic printer, and had other legal problems
as well. Perhaps Holorad has overcome those obstacles.)

Holographic recording methods are being developed to store massive amounts
of data throughout the volume of tiny crystals - the true-life version of the
standard Star Trek memory device. The information is converted to a 2-D array
of bits using a SLM and multiple such 'planes' can be stored within the same
holocrystal by varying the angle of the crystal relative to the optics. The
challenge now is to be able to do this in such a way that data can be added
without erasing what was there before!

Holographic methods may be used to optically sort data elements that form
building blocks for true optical computers and for data selection in optical
communications networks. These may be pregenerated and optically selected or
generated in real-time.

While there are significant differences in the details of the process needed
to produce those little logos compared to large white light holograms used for
marketing or 3-D volume images for medical diagnosis, the basic techniques are
similar and can be summarized very briefly. The following is the sort of
holography setup that is within the capabilities of a determined amateur:

A long coherence length laser. For amateur holography,
this is typically a polarized red helium-neon (HeNe) laser of 5 to 35 mW
or a green single frequency diode pumped solid state (DPSS) laser of up
to 100 mW or more. It may also be possible
to use some red diode lasers or even cheap red laser pointers, but not all.
See the sections below.

Some typical single frequency green (532 nm) DPSS lasers: Coherent
Compass 215M or 315M (up to 150 mW) or 532 (up to 400 mW). Not all green
DPSS lasers are single frequency though. Many of the cheap imports showing
up nowadays are NOT.

The output of the laser is passed through a beam expander and divided
into two parts. The higher the power of the laser, the
shorter the exposure - which as will be come evident, is critical.

Suitable lasers include medium to high power HeNe lasers

The reference beam is sent directly to the holographic film.

The object beam is sent to scene from the side so that it doesn't hit the
holographic film directly. Reflections from the objects of the scene fall
on the film producing an interference pattern with the reference beam.

The holographic film is extremely high resolution - 1000s of line-pairs
per millimeter (!!) since the finest detail that needs to be captured is on
the order of the wavelength of the laser light used. As with photographic
film in general, the higher the resolution, the slower the film. So, long
exposures or very bright lasers are needed!

Everything is mounted on an optical table that is made as stable as
possible since vibrations during the exposure must be minimized. Again, like
the resolution of the holographic film, the amplitudes of any unavoidable
vibrations must be kept well below the size of the wavelength of the laser
light. This usually means either a platform isolated from external
vibrations by air cushions, making it very massive, putting everything in a
sand-box, or all of the above. Several TON granite slabs are not unheard of
and basement locations are generally preferred to wobbly upper stories of a
50 story high-rise!

Once everything is set up, the studio or lab is darkened, the opaque
cover is removed from the film, and time is allowed to pass so that
any vibrations resulting from activity can die down. The laser is then
turned on or its beam unblocked for the several seconds or minutes in some
cases needed for the exposure.

The holographic film is developed in the normal manner. At this point,
the result looks like a black ruined film negative. The film is then
bleached (the silver is converted into a soluble compound and washed away)
leaving just the gelatin but with empty spaces where there were originally
silver grains. This provides enough phase contrast to recreate the original
interference pattern.

Under room light, the holographic plate just looks milky with perhaps a
hint of rainbow or diffraction effects. If you picked one of these off the
street without knowledge that it was a hologram, you would think it was just
a dirty piece of glass.

However, when placed back it the same location as it was during the
exposure with the scene objects removed and illuminated just with the
laser's reference beam, every aspect of the original scene reappears as if
framed by the window of the hologram plate.

(From: Joshua Halpern (vze23qvd@verizon.net).)

As far as a stable optical table goes:

You could buy one on eBay. You need to find one near you
otherwise the moving costs will kill you. Otherwise you need
to hire riggers. That costs between $500 to $1,000

There are two simple ways to build one. They start with the same
base, blocks of stiff styrofoam. The simplest thing then is to put a
block of granite or marble on top. Of course, you now have the problem
of attaching anything to the marble, but the table will be vibration
isolated

Otherwise get a thick piece of magnetic steel and put that on top.
you can then use magnetic bases. You buy the bases from a machine shop
supply place - not an optical supply house which would charge 2 to 3 times
the as much.

For even better isolation you put a sandbox on top of the styrofoam,
fill it with sand (there are grades, you want a grade with fine round
kernels). Put a sheet of plastic down before you float the steel/stone
on the sand and seal the plastic to the sides of the sandbox to avoid
having sand everywhere.

Note that all of these options involve moving something very heavy.
It will cost you money, but unless you have experience in doing such
things pay it. The money is a lot cheaper than the medical costs.

I haven't made holograms for a long time, but I started from the ground up. If
you've got $3K to play with, you can really start off very well. But if you
want to save money, you can build a complete setup for less than $1,000. It
may be far more advanced than what you may have intended, but you'll be able
to create pretty professional holograms.

The best bet is to get a 5-10 mW HeNe surplus laser for about $200 to $300
dollars. This type of laser should have a coherence length of at least 6" or
so. You'll also need some holographic film (I used to use Kodak stuff many
years ago -- don't know if they still make it but it was relatively sensitive
and easy to use). Next, you'll need to build a stable table. In a pinch, a
heavy wooden plank, slab of marble, etc., laid on a few partially inflated
inner tubes will probably be enough. I strongly recommend against a sandbox as
it's more of a pain in the ass to keep things clean and to prevent optics from
constantly shifting as you move things in the sand. Set the table up on the
lowest floor, preferably on a concrete foundation, to minimize
vibrations. Then you'll need to get some redirection mirrors and expanding
lenses. Finally, you'll need the chemicals to develop the exposed film.

From complete scratch, you are looking at an investment of about $350 to make
a simple hologram.

Here are more detailed suggestions:

Ditch the sandbox idea. While it does work, it's a pain to keep sand from
getting on all of the optics. Also, the light color of the sand means that
you'll often have to mask out stray reflections. Lenses and mirrors have a
tendency to shift when you move things around. I strongly recommend that you
build a solid, rigid table and place it on inner tubes. For my setup, I made
6 columns out of cinder blocks about 3 ft high. I then put down on top of the
columns a 2 inch thick pine plank measuring 4'x8'. I drilled six 4" holes in
the plank spaced evenly out and then placed 6 forklift inner tubes centered
around these holes. (The holes in the plank allowed for inflating the inner
tubes later on from the bottom to adjust the air cushioning.) On top of the
inner tubes I built a box out of wood measuring 4'x8'x4" inside
dimensions. Into the box I poured about 11 cubic feet of Redimix concrete,
using chicken wire and rebar for strengthening. The top was smoothed. After
five days of curing, I glued a 1/16" thick sheet of steel (4'x8') to the top
of the concrete. I painted the steel and the sides flat black. This was
definitely a very heavy, solid table that was not really intended to be moved
(except with dynamite!) Anyway, this might be more than what you'd like, but
the table performed exceptionally well. The height was such that it made for
comfortable working. The size meant I could do many intricate setups with
multiple beams. The steel top meant I could use magnetic mounts for the
optics. Total cost was less than $200 bucks in 1986.

Get good optics. I got most of mine through Edmund Scientific. They're a
bit expensive, though. All mirrors should be first surface (aluminum on the
front surface, not on the back). I recommend getting several mirrors of about
2"x2". You'll also need to get two or three in the 4"x6" range and higher.
You can never have too many mirrors. The lenses you'll need should mostly be
concave. Look for the largest diameter, shortest NEGATIVE focal lengths you
can find. These lenses will expand beams, which is generally what you'll be
doing in holography. I would try to get an assortment of -6 to -20 mm double
concave lenses at least 10 mm in diameter. If you don't use plate film, get
some clear glass plates about 4"x6" to sandwich the film. I built a special
jig that would clamp the film between the glass plates. Be creative, but try
to make the clamp jig as small as possible -- you don't want it to interfere
with any laser beams coming from behind the film to illuminate the object to
be holographed. Also in the optics category, you'll need to get at least 1
variable beamsplitter mirror.

Make or buy good optics mounts. You can go out and purchase optics mounts,
but talk about EXPENSIVE. My table had a steel top, so I built magnetic
mounts. The base of the mount was nothing more than a doghnut magnet (3 and 5"
in diameter). I solidly epoxied 3/8" steel rods to these magnets
vertically. Most were about 18" tall but some measured as much as 36" tall for
overhead illumination shots. The optics themselves were glued to masonite
pieces (with holes for the lenses). I used laboratory stand clamps to hold
the optics in place. They clamp to the optic mount rods and can swivel the
optics 360 degrees. Everything was painted flat black to reduce
reflections. I built about 16 mounts in all. Like mirrors, you can never have
too many.

Get the most powerful laser you can afford. I did most of my holography
with an 8 mW He-Ne laser that I purchased as surplus from Meredith
Instruments. The more power means shorter exposure times and better
results. You must get a TEM00 mode, single wavelength laser. I never tried a
diode laser, but I don't recommend them because the beam is not round like a
TEM00 laser. A good surplus HeNe laser will cost at least $300, but it's the
most important part.

Get the right film. Holography requires high resolution, special film for
the purpose. I'm not sure Kodak is still in the holographic film business, but
I had very good success with their film. I also used Agfa holographic film
with pretty good results. Check around on the internet for sources. There are
other types of media (e.g., dichromatic emulsions), but try films first. For
processing, I used Kodak D-19 developer and Kodak fixers. I used a bleach
mixture I made myself out of sulfuric acid (look in plumbing section of home
center for drain cleaner -- very dangerous stuff!) and potassium dichromate.
There are many other formulas out there so check around on websites.
Processing must be done under clean conditions in a dark room. You can use a
dim green safelight so that it won't exposure the red light sensitive film.
(Also see below. --- Sam.)

Though this is a long description, it should give you some ideas. There
are many books out there that should give you much more information. The
setup I described will cost somewhere around $1000. Once you've had some
success with making basic holograms, you'll probably invest in specialty
optics and other stuff to make more advanced holograms. With my setup, I was
able to do practically anything anybody else could do with equipment costing
many times as much as what my stuff cost. The key is to be creative not only
with the actual holograms themselves but also with the equipment you use.

Good luck and have fun.

(From: Rick Poulin (rpoulin@rohcg.on.ca).)

I used to be a holographic experimenter and got my supplies from Agfa but
sadly they got out of the business and left many people scrambling for a new
cheap source. If you want to pay through the nose,
Edmund Scientific or
MWK Laser Prodcuts are
the high water marks for pricing.

The difficulty of making holograms is *much* overrated. If you're not
going for commercial quality or for fancy stuff (image plane, rainbow etc.),
a simple Denisyuk (reflection) hologram can be made with *very* little
equipment (laser, lens, plate, chemicals).

With the right plate exposure time is in the seconds, not hours range;
and the vibration problem can be reduced with a robust setup like this:

I've been to a workshop (see below) which was held in a public building
next to one of the main thoroughfares in Stuttgart, where *everybody*
produced near perfect holograms, even the guy, not me :-), who carried
out a developed plate from the darkroom into near full sunlight.

Well, I ended up paying a lot of money for front-surface mirrors
and an AR-coated beamsplitter, and such like when I briefly (!!!) took
it up, but the plain fact is if you just want your first hologram, and
you have the film and developing chemicals, you just need:

Laser.

Lens to diverge the beam.

Object(s) to take hologram of.

Just put the lens right by the laser, get the beam nice and wide. Place
the (ideally glassy or transparent or translucent) objects somewhere in
the diverged beam. Put the film down-beam somewhere, so that the objects
are between the laser and the film. You may find that the objects cast
a shadow on the film; as long as a significant part of the film is not
in shadow, it should be ok.

Unless something moves really grossly, or you severely under- or over-
expose, you'll get at least some kind of a transmission hologram out of
it. (It won't be optimally efficient, but you really should see SOMETHING.)

Even if something moves (but not TOO much), you'll often end up with
a hologram that looks kind of stripy; the more movement, the more
stripes.

To view the hologram, just leave everything set up the way it was,
remove the object(s), put the hologram back in the film holder in
the SAME orientation in which it was exposed, let the laser illuminate
it, look THROUGH the hologram towards the laser at the place where the
objects were. You should see a holographic image of the objects. (I used
to cut a little corner off the rectangle of film at the top right, to help
get the film orientation the same when I wanted to view it. Then there
are only 2 ways to orient the film, rather than 8. Just remember whether
you had the emulsion side of the film facing towards or away from the
laser. (Put a corner of the film between your lips; the emulsion side
will feel sticky.))

(This is all in my limited experience; standard disclaimers apply.)

After that, you may want to try a 2-beam setup, with a reference beam
shining directly onto the film, and another beam illuminating the
object(s) but not the film. Then you can play with the relative
brightnesses of the beams, and get better interference, and therefore
a brighter hologram.

It gets harder when you want to produce reflection holograms, which
can be viewed in white light. You need more power, really, to get your
exposure times down.

Several companies provide all the equipment and materials needed to get
started in holography. One example can be found at the Arbor Scientific Holography
Page. Their prices may not be the best on individual pieces but the
convenience of one-stop shopping may outweigh the additional cost (except
probably for the laser especially if you opt to use a cheap laser pointer
for this!). Also check the various companies listed in the section:
New, Surplus, Walk-In, Mail Order, Kits/Plans
(Commercial).

The following is from a posting to the USENET newsgroup
alt.lasers in early 1999. I have no direct
knowledge of the contents or quality of these kits or whether they are still
available.

(From: Steve McGrew (stevem@iea.com).)

I've just received and tested the first shipment of a new
holography kit for education. It includes a HeNe laser, an optical
breadboard, adjustable mounts, dielectric mirrors, and a detailed,
understandable manual in good English (I helped with the translation).
The manual details a series of experiments and explanations that will
lead a student through all the basics of optics up through 3D
holography. The kit and experiments are designed for a college-level
optics course, but would be suitable as well for science enrichment at
the high school level. The kits are made in China under the
supervision of a university optics professor. Each kit fits neatly
into an aluminum suitcase. If you were to buy all the parts for the
kit in the U.S., they would cost somewhere in the range of $1,500.

My cost is $525 plus shipping; I'll provide these kits to any
bona fide school for my cost plus 10%, and will provide advice as
needed to teachers and students. (Price subject to change, so please
ask for confirmation of current price.)

Liti Holo, offers holography
related components and supplies for the DIY'er. Liti's main attraction
appears to be their instant no developing required holography plates.
These literally develop as they are exposed, so once the lights are turned
back on, the hologram (or lack thereof) is ready to be viewed.
(Liti Holo is a division of Liti
Holographics, a company specializing in the production of holograms
(including full color and motion) for marketing and advertising. At
least, that's what I gather from their Web site. There is no obvious
mention or links to Liti Holo.)

A single color holography kit using a red diode laser has been available
for awhile for around $100.
See Litiholo
Hologram Kit.

Now, a new version has been developed for making what they call full color
holograms. (Link from the page above. Whether it is generally available
- or ever will be - isn't
entirely clear.) The kit consists of a set of 3 lasers (all under 5 mW) with
battery packs, a holographic beam combiner, laser-cut plastic parts to
mount everything, and two boxes
of Liti's special instant no development 2x3" holography plates (20 total).
Now before you get excited, there are just a few limitations. :( :)
The most significant is that for the $250 to $300 price tag, it does NOT
include $10,000 laboratory single frequency lasers. What a surprise? ;-)
Due to the limited coherence length of the lasers, it appears as though
only essentially 2-D objects (like the tops of the bottle caps that are
provided) can be assured of being captured. This despite the spectacularly
3-D images shown on the Liti Holo Web site. So, if you thought this
was going to make nice 3-D holograms, probably not. For that, you'll
need lasers with decent coherence length. You might get lucky but more than
likely, you won't, or at least not very often. That's the bad news. The
good news is that this kit will make real holograms and the way things
are arranged, vibrations are less of an issue than with conventional
holography. So there's an excellent chance that your first exposure
will be successful. Taken together, these may be enough to get you
hooked. :-)

However, since the 3 color kit is a superset of the basic holography kit, it
is possible to make 3-D (single color) holograms if the limited dimensionality
becomes excessively boring. The red and blue lasers may have adequate
coherence length, at least some of the time.

What the instructions call "laser diodes" or simply "diodes" are either
actually diode lasers which have built in drivers (red and blue) or a
Diode Pumped Solid State (DPSS) laser (green).

These are all typical laser pointer type modules. To provide a divergent
beam, the collimating lenses must be removed. (This had already been done
on all but the red laser.)

Red diode laser

This is the physically smallest of the three. If the output looks like a
pointer beam, the front barrel must be unscrewed to remove the collimating
lens and spring (save for something else). Install the laser in the mount
and screw the barrel back on to help secure it. Add the "Special Clip"
(heat-sink) to the barrel once the laser is installed. Once set up, carefully
rotate the laser to maximize coverage of the spread out red beam on the
Holographic Plate Holder.

Wavelength: 638 nm.

Output power: Approximately 6 mW.

Spatial mode: Typical single mode diode.

Longitudinal modes: Goes through periods of multimode but appears to
settle more or less single mode after ~1/2 hour.

Useful coherence length: May be long for a decent percentage of
the time.

Green DPSS laser

WARNING: There is NO IR-blocking filter. Thus high levels of both 808 nm
and 1,064 nm are present in the output. To display anything on the SFPI
or to accurately measure the output power, an external filter must be added.

On my sample, the laser starts out very weak and requires 2 or 3 minutes
before it suddenly transitions to decent power. This appears to be related
to the temperature of the crystal as a great deal pump light leakage is
present initially, visible by eye.

The Special Clip (heat-sink) may be most important for the green
laser.

The beam from the green laser is fairly symmetric so orientation doesn't
matter. But it diverges slowly, so it will generally need to be positioned
farther from the setup than the red laser.

Longitudinal modes: Massively noisy multimode at first, then
settles down to 3 or 4 dominant modes. Sometimes, there will be a single
mode that may be 2 or 3 times higher than one or two others, but this is
not a situation that will remain for long.

Useful coherence length: The minimum is less than 1 mm but when
there is a single dominant mode, it could be very long.

Blue diode laser

A switchmode boost driver provides the 4 to 6 V required by the blue
laser diode. Once set up, carefully rotate the laser to maximize
coverage of the spread out red beam on the Holographic Plate Holder.

Wavelength: 445 nm.

Output power: Approximately 5 mW.

Spatial mode:: Typical single mode diode.

Longitudinal modes: Noisy multimode.

Useful coherence length: May be quite short most of the time.

Unfortunately, without actually monitoring the longitudinal mode structure
of these lasers continuously, it's a sort of crap shoot as to whether any
given exposure will be capable of significant depth without artifacts.

These are not $25,000 lab-quality lasers. Didn't I say that already? ;-)

Solder the battery boxes to the lead wires from the lasers,
or add real connectors.
Toss the clip leads into your electronics test department. :) Or, better
yet, substitute a well regulated low noise (linear) 3 VDC power supply
capable of at least 1 A for all the lasers. However, note that it must
be physically isolated from the holography table due to to need to avoid
vibrations from its power transformer and other components.

The fit of the tabs that secure the individual pieces of the various
assemblies is not very precise. If excessive force is required to push
a tab through a hole, trim or file it a bit. If very loose,
use some type of adhesive to stabilize assemblies like the "Laser Tower",
"Color Combiner Mount", as well as the ones for the laser diodes (but not
the piece that hold the actual diode laser since that has to be able
to pivot to adjust the beam direction). Make sure the battery boxes can be
removed to change batteries. (A slot may need to be filed to allow the
switch and/or wires to pass by the frame for the green one at least.)
DO NOT glue the "Stilts" to the laser assemblies as the stilts are not
used if making a single color hologram.

I used dabs of two-part Epoxy but RTV Silicone would probably be adequate.
DO NOT use stuff like Superglue (cyanoacrylate) or Duco Cement, which may
eat the plastic. 5-Minute Epoxy can usually be removed if necessary
without collateral damage.

Put some rubber feet under the Laser Tower and Plate Holder so prevent
them from moving around too easily.

Take extreme care in handling the lasers as the PCBs attach directly
to the leads of the laser diode and flexing can result in breakage. In
addition, the "Green Diode Laser" is actually a Diode Pumped Solid State
(DPSS) laser (not a simple diode), and flexing may loosen the mounting of
the IR pump diode resulting in a loss of alignment and degraded performance -
unstable or no output.

The "Green Diode" (see above) has NO IR-blocking filter, so there is
significant mostly invisible light at 808 nm from the pump laser
diode. This may even be visible as a faint red glow.
Do not stare into the front of the Green Diode
even from an angle as the IR spreads much faster than the green light.
There is also some IR leakage at 1,064 nm (twice the wavelength of the 532 nm
green output), but this follows the green beam path so avoiding that
will also avoid the 1,064 nm IR.

The Color Combiner (Holographic Optical Element, basically a pre-made
speical hologram) is bonded to a piece of glass. Do not attempt to peel
it off - damage will result.

Use a spring clothes pin or something similar to secure the Blue diode
assembly to the Laser Tower once the optimal position has been determined.

When preparing to expose, remove the film plate from the box in total
darkness to avoid fogging the unused plates, but use the dimmest safe light
that will allow you to avoid knocking everything over to position the plate.

While waiting for vibrations to die down, turn off the safe light.

When ready to expose, remove the black card/shutters as carefully as
possible to avoid introducing new vibrations. The laser light shining
on the black paper may be enough to do this without a safe light.

Stand perfectly still or sit perfectly still and count out or time in
some other way for the 5 to 10 minutes for the exposure.

My first hologram was of the top of a cell phone main PCB (a Casio G'zOne Rock
if you must know, found along the side of the road). While I would classify
this as a success - sort of - there was almost no blue and the contrast was
rather poor (or at least poorer than I had expected). However, it was sharp
and showed enough depth to be able to clearly see the limited 3-D nature of the
circuit board components.

The lack of blue was either due to the need for more blue and less red/green
in the color balance mix, or bad luck on the blue laser not being very
coherent (or changing coherence) during the exposure.

Unfortunately, while the instant plates make it easy to make holograms,
that also makes it easy to expose plates and end up with nothing on them.
And the plates are not inexpensive, $3 or more for each 2x3" plate.

If you ask most laser 'experts' about the possibility of using a laser pointer
or inexpensive diode laser module for making holograms, the typical response
will be to forget it - the coherence length is only a few mm and therefore
inadequate. This apparently isn't the case. The coherence length for a
typical laser pointer or diode laser module may actually be more like 200 mm
(10 inches) - comparable to that of an HeNe laser and, with care, will remain
stable for long enough to make an exposure. While it may be unreasonable to
expect any old $8.95 laser pointer to produce the same quality results as
a $500 HeNe laser, surprisingly good holograms can be obtained on a budget.
And, it would appear, that in some cases, they can actually be superior.

While I don't know how to select a laser diode to guarantee an adequate
coherence length, it certainly must be a single spatial (transverse) mode
type which is usually the case for lower power diodes but those above 50
to 100 mW are generally multimode. So, forget about trying to using a 1 W
laser diode of any wavelength for interferometry or holography. However,
single spatial mode doesn't guarantee that the diode operates with a single
longitudinal mode or has the needed stability for these applications. And,
any particular diode may operate with the desired mode structure only over
a range of current/output power and/or when maintained within a particular
temperature range.

I had my fingers crossed tighter than ever for this one -- moving up to
35 mW of power for holography using a diode source. It worked!

The module used contained the Hitachi 35 mW, 658 nm diode, along with
AR-coated anamorphic prisms (optional) and high-grade collimating optics.
The measured optical output after collimating optics is 27 mW and total cost
for putting the whole thing together was about $50 to $60.

This little baby exceeds the performance of any HeNe in its power range,
including the $5,000 Spectra-Physics at 25 mW.

Those diodes are real little buggers once they're set up with an
interferometer. Very strange behavior (at least strange after working
with gas lasers for so many years) - and in a good way.

In any case, this baby is ROCK solid. The final test which put us over
the top was so incredible that I thought there was something wrong with
the set-up. I would tap on the table just to make sure. It's almost as if
a fringe-locker was in place. Even with the best HeNe that I've had here
(Spectra-Physics 124B Stabilite) there would ALWAYS be some "drift" or
what I call "float". (Float is the feeling that fringes are not entirely
still -- it's not something that shows up very clearly to the eye.
It's more of a "feeling" when testing). The fringes with the new diode
are locked so tight it's almost like watching a still photograph.

As far as the coherence length is concerned, I measured (using a Science
and Mechanics PhotoMeter placed in the fringes) out to 14 feet without any
change. As you may know, this amount of coherence would require a rather
expensive etalon on any lab laser. Up until this point, we were only capable
of recording a few inches using diode lasers.

This diode created two very bright test holograms that exhibited depth all
the way back with the object(s) (1. ocean coral, 2. angel statue with
wings). For a special twist, I used an initial set-up for a 30 x 40 cm
hologram and then just shot two 4 x 5s with the set-up as-is. Even though
the size of the holograms are 4 x 5, they will give you an indication of
what a 30 x 40 cm hologram would turn out like -- since your beam spread,
exposure, etc. are calibrated for that size.

For a complete report, along with photos of the module, the holograms, the
visible beam in my lab and a interesting size comparison to a Spectra-Physics
124B HeNe laser go to the
Our Own 25 mW Laser
Page. (There are also other reports preceeding this one which may be
accessed at the Holoworld site.)
D and S Lasers is a spinoff
of Holoworld offering plans, a kit, as well as an assembled 25+ mW diode laser
system with long coherence length suitable for holography.

As for using green laser pointers, realize that these are based on an entirely
different technology than laser diodes in red pointers. Green pointers are
Diode Pumped Solid State (DPSS) frequency doubled lasers. To be useful for
holography, a laser has to have a decent coherence length. For a short cavity
laser like a that in a laser diode (a fraction of a mm) or green laser pointer
(2 to 10 mm typical), this implies single longitudinal (and of course
single transverse) mode operation. Some red diodes do this under some
conditions (by controlling diode current and diode temperature). Depending
on the specific configuration of the laser cavity in a green laser pointer,
some may also operate single mode. Maybe. But, stabilizing them without
major modifications may be difficult. The CASIX DPM crystals generally do
not operate single mode but may do so at times depending on pump
power and pump beam alignment. A discrete cavity pointer laser will likely
operate single mode up to a modest power level and then switch to multimode.
Many or most green pointers are now quasi-CW and/or Q-switched which further
complicates matters.

(From: Colin K. (colinholo@yahoo.com).)

Laser diodes do work. I would not say they work well. At least the APC style
most amateur holographers use. There needs to be a method of locking the
frequency to single mode. If you only need 5 mW then Integraf has a very
reliable diode for $35 with a coherence length of more than 6 ft. I run one
from two D-cell batteries and have made more than 30 holograms with it with
no failures. As the red diodes increase in power it becomes increasingly
hard to get the line to stabilize. I have a TEC based laser with the
Panasonic 50 mW diode and I have had much difficulty keeping it in a single
mode. When I can the coherence length is quite long. More than 12 feet.

The 35 mW laser Frank sells from the Holoworld site (APC with Mitsubishi
Diode) makes a good hologram most of the time but it will run in multiline
mode at random times.

The best laser I have found in red is the
Analog
Technologies TLM-S1 Tunable Laser Module but it's not cheap (don't ask!).
There is also a less expensive non-tunable laser that will be available for
about $800 very soon. I am hoping to test a sample with a 50 mW diode in a
few days. The 25 mW has extremely long coherence lengths.

(From: Tony (kilm02nspm@clara.co.uk).)

I thought that laser diodes would be unsuitable for holography due to their
supposedly very short coherence length until 1999, when I read of holograms
being made using laser pointers. I didn't believe it, but thought it wouldn't
hurt to try. I bought a laser pointer (the bullet style with light
feedback regulation), broke it open and fixed the diode and board to
an adjustable mount, powering it from 3 AA cells. It worked first
time, producing brighter holograms that were ever possible with my old
1 mW He-Ne. Having only a small table I've never been able to confirm
the long coherence lengths quoted by some but I have found reflections
from objects at the back of the table, giving a coherence length
(taking into account the path difference there must have been) of at
least 50 cm. I tried a few pointers and found only the cheap
no-regulator types with only a resistor and diode don't work. One
thing to remember is they do need to warm up just like a gas laser so
don't expect to click the power on and off for an exposure - it's
still best to use a shutter. Set up an interferometer to check the
warmup time as well as you table's stability.
The simplest way (assuming you've already built a vibration damping
table) to make a transmission hologram with a diode laser is: Remove
the collimating lens from the pointer, this produces a 'stripe' of
light which can be used instead of a beam expander. Screen off the
edges of the stripe next to the laser until only your objects and
reflector are illuminated. With the laser at the left centre for
example, you would place your object below centre of the right side
and your reflector for the reference beam above centre on the right.
Arrange your plate at the bottom of the table, the fun part being to
keep it out of the direct beam while facing the reflected light from
your object and being fully illuminated by the reference beam at the
correct angle. You'll have to use some white card in the plate holder
to try and balance the light from the object and reference beams. All
this is much easier with more mirrors of course but for a zero-budget
experiment it does work. You can make a partial reflector for the
reference beam by painting a piece of 6 mm glass black on one side and
roughly control the intensity by moving it nearer or further from the
plate or film.

While some laser diodes are particularly good for use in holography and
interferometry due to their natural tendency to operate in single spatial
and longitudinal mode, many others can be convinced to behave by a combination
of current and temperature tuning. However, some means is needed to
check for mode hopping and multimode operation. This can be done with fancy
and expensive instrumentation this is normally out of reach for even the well
equipped holographer. There are low cost alternatives which provide some
of the same information.

(From: Jonathan Head (holosjmh@primus.ca).)

Here's my problem - laser diode frequency stability. It used to be the
holographer's biggest issue was vibration stability. Now it's frequency
stability, at least if you use a laser diode. And given the huge advantage
of coherence length, robustness, and lower cost over HeNe lasers, who wouldn't?

I'm building a heat sink for it. A TEC maybe later, right now I'm going
low-tech. I believe I can keep the temperature quite low (close to 0 C)
and stable enough to shoot between mode hopping episodes, with the design (by
Colin Kaminski) I have. I have run numerous monitoring tests with my
interferometer and audio detector (solar cell/amp/headphones) which, believe
it or not, (at first I didn't) can actually (and cheaply) detect mode hopping
via the amplitude shifts in the beam. They are audible clicks, which turn
into static when the diode starts into multi-mode operation between mode hop
free temperature zones. The beam quiets down when solidly in multi-mode,
then the static returns followed by dwindling clicks, as it transits to the
next temperature zone.

You can therefore easily detect mode hopping *without* an interferometer at
a cost of only about 8% of the total beam diverted to the solar cell using
a glass plate beamsplitter. A single beam will do. I've placed the BS
before the shutter and can use it to time my hologram exposures. But I
digress. Although since I haven't found this in your FAQ I thought I'd
mention it.

For some time I've been running tests with an interferometer in
conjunction with the audio set-up mentioned above. The
correlation between the two (audio and visual) is interesting and useful.
Primarily I'm testing methods to control the temperature of the LD, and
monitor its mode hopping and linewidth behavior, without the benefit of
expensive instruments. (I think holographers have enough expenses just from
the film, plates, optics, and time away from family.)

The first report I saw of the possibilities came from Tom Burgess, who
posted on Frank DeFreitas' holography forum that he noted clicks, and rasps,
in the beam that he thought might be mode hopping since they were
accompanied by jerks in the pattern, and fringe washouts, respectively.
This turns out to be the case.

It helps to have an interferometer set up simultaneously to observe beam
activity, but it isn't strictly necessary (and quite impossible if you are
shooting a hologram).

It's been previously shown that there is a correlation between noise in the
total intensity of the beam, and mode hopping. The solar cell will pick this
up as output intensity fluctuations directly caused by the laser switching
between wavelengths. A "click" is heard for each discrete mode hop, and
sometimes the mode hopping is quite rapid, which results in a "static" like
sound of various tempos and sound levels.

In addition to a small silicon solar cell, all you need is an amplifier
equipped with phono jacks, a short RCA cable and headphones. A photodiode
would work also. Using a plain piece of glass and perhaps a transfer mirror
or two, divert part of the beam directly to the solar cell, which is
connected directly to the amp inputs. Listen via headphones or you may get
feedback interference from external speakers. I'd also recommend diverting
the beam before the shutter, so that you can monitor the beam before an
exposure. Once you've established that a background hum can be interrupted
by blocking the solar cell with your hand, you can then attempt to "listen"
to the beam for various manifestations of mode hopping activity.

This is a practical means, especially for holographers on a budget, to
determine suitable windows of opportunity in which to make their exposures.
The wavelength stability of laser diodes depends on temperature and
injection current, among other things, and unless these two factors are
strictly controlled there will always be a chance for mode hopping to ruin
an otherwise good hologram.

The absence of audible indications (clicks and/or static sounds) will not,
however, guarantee that the LD is operating in single mode, or at least with
a narrow enough linewidth, to make a good hologram. This is because there
are also times during multimode operation when no mode hopping occurs,
and/or the intensity fluctuations are out of range to pick up. I've found
this occurs as the diode moves through a zone of instability, of which there
are many, determined by the particular combinations of case temperature and
current. The audible indications occur as the LD enters and exits an
unstable zone. In the middle of the unstable zone, it is often quiet (even
while fringes are completely washed out). Therefore, one can determine where
the laser is operating fairly easily, by simply monitoring the situation.

This should save a significant amount of wasted film for those holographers
using a bare-bones laser diode. For anyone using a TEC this is a way to find
the zones of stability, and establish favorable set points.

To create a useful holographic display of a moving scene requires an almost
unbelievably large amount of data processing and throughput. Suppose you just
wanted to produce a holomovie of a 50 x 50 x 50 cm volume using a 50 x 50 cm
display device. Given that your typical holographic film must have a
resolution on the order of a wavelength of the light used to create/reconstruct
the hologram - 1,000 line pairs/mm or better - this would mean that some sort
of spatial light modulator (e.g., LCD) would be needed with a similar
resolution to reproduce moving images. That implies over 1.25x1012
or 1.25 Terapixels! And you thought high resolution laptop screens were
expensive! To make things easier, we'll assume 1 bit per pixel for the
interference pattern, resulting in 100 Gbytes per frame! To provide smooth
motion, one needs a minimum of 24 to 30 fps so you are looking at 2.4
Terabytes/second. Now, granted, various compression techniques (e.g., MPEG-26
by then) can be used to reduce this by perhaps a factor of 10 to 100 or more
(and no doubt such processing will be much more advanced once this sort of
folly becomes at all practical) but that is still 24 Gbytes/second through the
communications channel. Hmmm, that doesn't look quite as impossible! This
doesn't take into account the need for color but at least the laser(s) will
probably be the least of your problems in bringing such technology to market!

Such a display is simple in principle:

To write a hologram, scan an electron or laser beam across a material
(the SLM) that changes some property (e.g., density, color, phase) with
the interference pattern. A sufficiently high resolution LCD would also
work (see below).

To display or read out the hologram, shine an low intensity expanded laser
beam through the pattern as with a normal hologram.

To erase the hologram, either overwrite it or shine a higher intensity
laser beam or 'flood' electron beam on the material.

I was actually discussing stuff like this (in a former life) in the early 1980s
realizing that either a dedicated special purpose computer or something as yet
non-existent would be needed to achieve any sort of througput.

That is still the case.

However, for stationary images (e.g., medical visualization where one wants to
view anatomy from various angles with proper perspective, etc.), the speed may
not matter as much as long as writing doesn't take more than a few seconds.

So let's see.... For a 10 cm x 10 cm SLM, resolution order of a wavelength
of visible light, that's only about 50 billion pixels. Not your ordinary
CRT electron gun - more like a scanning electron microscope. A few 10s of Giga
bytes per second (for a 1 second refresh rate) is the same order of magnitude
as the internal memory busses on some of the latest microprocessors, so no big
deal. :) Of course, then multiply that annoying frame rate thing. ;-)

A search of a patent database at using keywords like "Three Dimensional
Display" and "Holographic" should turn up a variety of interesting, though
probably for the most part unrealistic (as yet) approaches to this problem.

(From: Steve Roberts.)

The problem is twofold, resolution and bandwidth. Resolution, because a
hologram needs far more sensors per mm then available CCDs can provide, and
bandwidth because only a dedicated direct array of fiber optic lines could
handle the bandwidth. Your not going to see the scene shot with actual lasers.
A computer and two or more cameras will be used, to synthesize the data.
Experimental small scale displays have been made at low resolution, but the
Cray computer they used to do the calculations is not something I'd have room
for in my living room. Laser beams loose coherence after a short distance, so
the guys at Monday Night Football aren't going to go blind, as lasers will not
be used to gather the images. Maybe at the end of my lifetime in 30 years, but
not any time soon.

(From: James Hunter Heinlen (dracus@primenet.com).)

There have been a few made. Right now, the only applications that can afford
such tech is very high end medical, and government, mostly military, but I
believe the DoE has one in their nuclear power simulation program. At any
rate, they are fiendishly expensive, and the one I saw (when I was still
doing consulting in the explosives industry) used a couple of Cray YM/P-2E's
(when they were new) as signal processors, plus other computers to do the
modeling, run the simulation, and produce a real time data stream to use as a
signal to be processed. It was considered the low end of the tech, and
produced a dim (but beautifully detailed) 3D moving image of whatever you
wanted in real time. They were using it to display the progression of a
shock-wave through multiple layers of (non-ideal, realistic) rock in fine
detail. We had to turn off the lights to see the display. The 'monitor'
looked like a plexiglass fish tank. If you want more info, there was a
couple of good articles about the displays in Government Computer News when
they first started making this type of system.

(From: Ted (email address N/A).)

There have been a few attempts to display true interference pattern
holograms created by lasers on very high resolution LCD displays. I was at a
digital imaging conference and they had one there. The screen itself, I
think, had about 50,000 x 50,000 pixels. The actual holograms were scanned
by a drum scanner at 90K x 90K pixels each and displayed at 1:10 (or
something like that) on the screen, which was about 17"! The hologram was
very bright, more brilliant than most I've seen on film. The spokesman said
each hologram file took well over 100 MB.

Note our eye process signals at about 27 fps, so about 30 fps is needed. At
30 fps, a one-second holographic animation of such would be 3 GB! An hour
would be 180 GB+. Clearly, even true hologram motion, is still a long way.
Artificial interference holograms created by computers would require even
more storage and processing power. But, at the rate things are going in the
computer industry, it is highly feasible in 10-20 years this could become a
reality.

(From: Andre de Guerin (mandoline@gtonline.net).)

There is a new type of liquid crystal display that generates a hologram
directly by producing the interference patterns on the surface of the LCD then
illuminating it with visible light.

The display this produces is a moving 3-D hologram in real time.

One slight problem... The LCD density is something ridiculous like
3,800 x 3,800 pixels with a pixel size of 10 um x 10 um. There would be
major problems with mass producing this sort of display, given that standard
1280 x 1024 laptop screens 1/10th the size have problems with dead pixels.

(From: Sam.)

Actually, there are bit more than one slight problem, not the least of which
is that the resolution cited is at best marginal and feeding it with data must
be a real treat, bandwidth and processing-wise! :) However, dead pixels,
at least, would not be a major problem, just adding a bit to the background
noise since localized defects in a hologram do not appear localized in the
3-D reconstruction.

Here is a summary of the types of lasers with a long enough coherence
length to be used to make holograms of macro-objects (more than a
fraction of a millimeter in depth). See the relevant chapter on
each type for more information.

Helium-Neon (HeNe) lasers:

Most polarized HeNe lasers can be used to make decent holograms if allowed
to warm up for a half hour or more. A spatial filter will probably be
needed (or at least highly desirable) to clean up the beam. The
best will be large frame lasers like the SP-124 and SP-127, but internal
mirror lasers in enclosed laser heads should be nearly as good with adequate
warmup. HeNe lasers produce at most 40 or 50 mW at 632.8 nm (red-orange).

Argon or krypton ion lasers:

These have relatively short coherence length unless fitted with an etalon for
single frequency operation. However, even a single wavelength without
etalon argon or krypton ion laser may be acceptable for some holography work.
The most useful (highest power) lines would be 488 nn (blue) and 514.5 nm
(green).

Diode lasers:

I don't have specific recommendations but some should be excellent if
temperature and current controller. Check on the holography Web sites
or ask on the holography forums, below.

Diode Pumped Solid State (DPSS) lasers:

These must be single frequency to be useful for general holography. This is
partially due to the typically short cavities of (non-ring) DPSS lasers,
and the wide gain bandwidth of most SS lasing materials. DPSS
lasers are mostly 532 nm (green). But there are some blue ones at 457 nm
or 473 nm. Specific DPSS lasers I know to be excellent for holography are:

Coherent, C315M (green, up to 150 mW) and with a bit of work, C215M
(green, up to 75 mW).

Coherent 532 (green, up to 400 mW).

Coherent Verdi (green, up to 18 W). :)

Uniphase uGreen. (green, up to 50 mW). The best will be the 4601, 4611,
4711, and others that have two TECs.

Melles Griot 58-GSD-309 (green, up to 3 W), 58-BLD-605 (blue, 457 nm, up to
400 mW), and others in the same family. There are also lower power Melles
Griot DPSS lasers that should be suitable.

I don't know of any new inexpensive DPSS lasers that are consistently
acceptable for holography.

Pulsed Solid State (PSS) lasers:

Holograms can be made with some PSS lasers. While cavity lengths for
PSS lasers are usually relatively long (at least compared to most
DPSS lasers), the wide gain bandwidth of the SS lasing medium will
mean many longitudinal modes are able to coexist unless specific
means like an etalon are included inside the cavity.
I don't know of any specific
commercial PSS lasers to recommend, but with minor modifications,
the widely available small
rangefinder laser, SSY1, has been used successfully for holograms.
See the section: Using SSY1 to Make
Holograms.

The Coherence Length (CL) of the laser is a critical parameter determining
the usable depth of an object. This is the Path Length Difference (PLD)
between the reference beam and the reflection of the object beam from any
feature on the object. An ideal single frequency (Single Longitudinal Mode,
SLM) laser would have an infinite CL; a true white noise "laser" would have
a CL of 0. There have been extensive discussions on this topic on the various
Internet forums like Photonlexicon without any definitive conclusions. Here
is my take. Comments welcome.

The following was written specifically for the HeNe laser but should apply
to most other single spatial mode (TEM00) lasers.

Any laser using a Fabry-Perot cavity like a common unstabilized HeNe will
have a Coherence Period (CP) equal to the tube length. The CP is the Path
Length Difference (PLD) over which the fringe pattern repeats (or almost
repeats subject to reduction in fringe contrast due to noise).

The useful coherence length is something else.

A Single Longitudinal Mode (SLM, "single frequency") stabilized laser like
a C315M or stabilized HeNe will not have a CP or one that can be considered
to be infinite. Its Coherent Length (CL) will be extremely long and related
to c/uncertainty (of optical frequency where c is the speed of light) in
the stabilized line position during the exposure, but never less than about
c/(Gain Bandwidth) (GB, 1.5-1.6 GHz for the HeNe laser).

A two mode laser with the modes stabilized in a fixed position or with a
short exposure such that they don't move very much should have a better
CL than a similar unstabilized laser. For example, a tube with a 20 cm
cavity length for a 750 MHz mode spacing lasing on 2 modes with an
instantaneous exposure would have a CL of 40 cm.

Any medium to high power unstabilized HeNe with an exposure time long
enough for more than 1 complete mode sweep cycle will tend to have a
similar CL of c/GB. So a Melles Griot 05-LHP-151 (~5 mW) and an SP-125
(~50 mW) should produce similar results for a long exposure. (CL=20 cm.)
For a short exposure, a shorter laser may have a slight advantage, similar
to the two mode laser, above.

Due to mode competition effects, really short tubes may have a greater CL
under the same conditions due to mode hops occurring higher up the gain
curve so that the effective span of the lasing lines even for a long
exposure is smaller. For example, the effective GB may only be 1.2 GHz
resulting in a CL of ~25 cm.

The major conclusion from this would be that the advantage of a long cavity
is offset by the increased number of longitudinal modes with significant
power so that CL remains similar. The effects of the weak modes on the tails
of the gain curve may be small for a holographic exposure so there may be
some advantage to a long laser other than power.

Holography Sites in my Bookmark File
for links to many holography Web sites.

There is a weekly holography show on-line at
Holotalk which
has feature stories and special guests by hosted by
The Internet Webseum of Holography.
You may need special speech/video plugins for you browser to take advantage
of this Web page.

Laser Communications

The term 'laser communication' can mean many things but generally refers to
the transmission of information via a laser beam in free-space or a fiber-optic
cable. A laser communications system must then consist of:

Laser source - This can be CW or pulsed. Common types are helium-neon and
argon ion gas lasers and nowadays, IR diode lasers.

Modulator - A means of impressing the information on the laser beam. This
can be via electrical control of the laser power itself, or through the use
of a mechanical, acousto-optic, or other external device. Many forms of
modulation coding are used including amplitude, phase, frequency, PCM, PAM,
and others. See the section: "Modulation and
Deflection.

Transmission medium - The main distinction is whether it is free-space
(e.g., air, water) or guided (e.g., fiber-optic).

Detector - A means of sensing the optical transmission and converting it
to an electrical signal. Common types are PIN photodiodes and
photomultiplier tubes (PMTs).

Demodulation - Extracting the actual information from the electrical
signal.

For more information and discussions on amateur laser communications, join the
Laser Reflector. It is
run by ham radio operators who do long distance free-space communications. One
is working on laser EME (Earth-Moon-Earth), and another is into
non-line-of-site weak signal operation using low baud rate long term
integration and advanced DSP techniques with coherent signals!

The Laser Reflector Web site provides archives of past discussions indexed by
date (year and month) and a large set of links to other laser and laser
communications sites.

Offers of inexpensive lasers, laser components, and other related items also
appear from time-to-time via this email discussion group.

Anyone with an interest in laser communications is welcome to join. You don't
need to be a ham radio operator. Just send email to majordomo@qth.net with
'subscribe laser' (without quotes) in the message body.

Not surprising, the potential of optical communications was recognized by
researchers even long before the laser was invented. The following is just an
example of how easy it is to turn a laser that can be modulated and solar cell
into a line-of-site comm link. This was just an ad-hoc experiment but

Bell Labs may have actually developed and produced some number of portable
demonstrators to promote the idea of optical communications. The typical unit
appears to have consisted of a HeNe laser tube, power supply, and modulator,
along with a separate receiver based on a solar cell, all packed in a handy
traveling salesman's type sample case. :) I say "may have" and "appears"
because I can't quite tell from the limited information and photos I have if
it actually had a working laser or just a cool-looking neon sign-type tube
for show - and actually did the communications with a separate conventional
modulated lamp (an arc lamp is mentioned in the description I have and its
presence doesn't make much sense otherwise). In any case, laser or not, this
unit was used in community relations and school programs to show how telephone
signals could travel over an optical beam. Some photos of one of these units
rescued from the dumpster can be found in the
Laser Equipment Gallery (Version 1.76
or higher) under "Assorted Helium-Neon Lasers" (giving it the benefit of the
doubt in actually containing a laser!).

(From: George Werner (glwerner@sprynet.com).)

Back in the middle 60's our group at Oak Ridge National Laboratory had
built a HeNe laser for the purpose of demonstrating to interested groups.
One time when I had brought it home in preparation to taking it "on the
road" I decided to test its long distance transmission. For distant
transmission we used a beam expander which was half of an 8x binocular with
a 30 mm objective. We also had built into our power supply a jack into
which we could plug in an audio modulation. I set up the laser on the
kitchen table near a window with a little pocket radio supplying a signal
to the modulator from the local radio station. With a mirror I directed
the beam out the window and across the valley to the parking lot I could
see where the city maintenance department has a number of vehicles parked.

It was about a mile away. Looking with another telescope I could see that
my beam was getting there when it retro-reflected from a car's tail light.

Then, taking with me a Fresnel lens and an audio amplifier attached to a
solar cell, I drove over there to see what it looked like up close. This
was at about 5:30 in the afternoon, still bright daylight, so the red spot
was not obvious, but I soon found it. About that time the night watchman,
as he should, came to see what it was about. I explained that I was
checking on this light that I was beaming down from halfway up the hill
across the Turnpike. He looked in that direction but didn't see anything.
Where he was standing, the beam was landing between his belt and his
shoulders. "You'll have to scootch down a little bit to see it," I said. He
found this hard to believe but he tried it and there was no mistaking there
was a light. I would compare it to the brightness of a locomotive
headlight about a half mile down the track at night (except that it was
red).

Then I put my 18 inch f/1 Fresnel lens in the beam and put the solar cell
at the focus (now bright enough to see the reflected light) and the radio
station came through loud and clear. With a Polaroid camera I photographed
the light coming from my house. Shot from that distance, all the houses
are very tiny, but magnification shows a white blob where my house should be.

P.S. I did not get arrested for trespassing. :)

(From: Sam.)

Although George was definitely not an amateur in the laser field of the day,
this could very well have been the earliest (or at least one of the earliest)
examples of amateur laser communications since it I bet it wasn't part of his
job description!

Miscellaneous

Of course you can't reach the stars but there may be enough scatter in the
air to show the direction. :)

(From: Louis Boyd (boyd@apt0.sao.arizona.edu).)

In my experience a 5 mW red laser does not do the job unless there's a
lot of dust or water droplets in the air. The problem is the dark
adapted human eye is very insensitive to red. Also backscatter from
small particles is reduced as wavelength increases. I can't give a
specific power level because it's so dependent on the particles
suspended in the air. Under the right conditions a 3mw green pointer
would be easily visible for a few people standing together but probably
won't be adequate in very clean air. Blinking the laser can make it
easier to detect and reduce power consumption. You also didn't state
the size of the group. The distance of the observer from the emitter
makes a difference.

The "vanishing point" for off axis viewer isn't at infinity and is
dependent on the power level and the hight of suspended particles. The
effect is that what you are pointing at may not be exactly where other's
perceive the end of the "beam" to be. You may actually be better off
with a larger beam diameter using a modified flashlight with a halogen
bulb.

One of the more powerful "MagLight" or "Surefire" flashlights with a an
extension of a couple of feet of ABS plastic with internal baffle rings
to prevent side scatter does a good job. This can put out around a watt
of light and it's a lot cheaper than an adequately powerful laser. If
this is for a large group get one of the "million candlepower" lamps and
make the baffle out of a "honeycomb" of tubes with black flocking blown
into them. Those have over 10 watts of light output. If you need to do
this for a large crowd like a stadium use a xenon short arc lamp
spotlight with hundreds of watts of output.